dovecot: security backports, new versioning scheme

- Fix dovecot package download URLs for new
  [release](https://github.com/chatmail/dovecot/releases/tag/upstream%2F2.3.21%2Bdfsg1-3%2Bchatmail2)
  with
    - debian-security backport for 12 CVEs
    - distro-specific suffix (+deb{release}u1), enabling a simplified
      primary URL path and combined github releases
- Use VERSION_ID from os-release as deb_release instead of codename
  mapping, reorder hash-dict to match Github release page
- Remove redundant parsing/validation, let function validate against hash dict
- Update test expectations and test new versioning derivation

.github/workflows/ci.yaml

@@ -29,7 +29,7 @@ jobs:
           ref: ${{ github.event.pull_request.head.sha }}
           persist-credentials: false
       - name: download filtermail
-        run: curl -L https://github.com/chatmail/filtermail/releases/download/v0.7.1/filtermail-x86_64 -o /usr/local/bin/filtermail && chmod +x /usr/local/bin/filtermail
+        run: curl -L https://github.com/chatmail/filtermail/releases/download/v0.7.4/filtermail-x86_64 -o /usr/local/bin/filtermail && chmod +x /usr/local/bin/filtermail
       - name: run chatmaild tests 
         working-directory: chatmaild
         run: pipx run tox

.github/workflows/docker-dispatch.yaml

@@ -8,7 +8,7 @@ name: Trigger Docker build
 
 on:
   push:
-    branches: [main]
+    branches: [main, j4n/dovecot-multidist]
     tags: ['[0-9]+.[0-9]+.[0-9]+']
   workflow_dispatch:
 

.github/workflows/docs-preview.yaml

@@ -51,7 +51,7 @@ jobs:
           mkdir -p "$HOME/.ssh"
           echo "${{ secrets.CHATMAIL_STAGING_SSHKEY }}" > "$HOME/.ssh/key"
           chmod 600 "$HOME/.ssh/key"
-          rsync -rILvh -e "ssh -i $HOME/.ssh/key -o StrictHostKeyChecking=no" $GITHUB_WORKSPACE/doc/build/ "${{ secrets.USERNAME }}@chatmail.at:/var/www/html/staging.chatmail.at/doc/relay/${STEPS_PREPARE_OUTPUTS_PRID}/"
+          rsync -rILvh -e "ssh -i $HOME/.ssh/key -o StrictHostKeyChecking=no" $GITHUB_WORKSPACE/doc/build/ "${{ secrets.USERNAME }}@chatmail.at:${STEPS_PREPARE_OUTPUTS_PRID}/"
         env:
           STEPS_PREPARE_OUTPUTS_PRID: ${{ steps.prepare.outputs.prid }}
 

.github/workflows/docs.yaml

@@ -47,5 +47,5 @@ jobs:
           mkdir -p "$HOME/.ssh"
           echo "${{ secrets.CHATMAIL_STAGING_SSHKEY }}" > "$HOME/.ssh/key"
           chmod 600 "$HOME/.ssh/key"
-          rsync -rILvh -e "ssh -i $HOME/.ssh/key -o StrictHostKeyChecking=no" $GITHUB_WORKSPACE/doc/build/ "${{ secrets.USERNAME }}@chatmail.at:/var/www/html/chatmail.at/doc/relay/"
+          rsync -rILvh -e "ssh -i $HOME/.ssh/key -o StrictHostKeyChecking=no" $GITHUB_WORKSPACE/doc/build/ "${{ secrets.USERNAME }}@chatmail.at:"
 

CHANGELOG.md

@@ -1,5 +1,60 @@
 # Changelog for chatmail deployment
 
+## [1.12.0] - 2026-07-31
+
+### Breaking Changes
+
+- [**breaking**] Introduce configurable system limits to reject new address creation and limit imap/smtp connections.
+  Dovecot default connection limit lowered from 50k to 10k,
+  Postfix default connection limit lowered from 5k to 1k,
+  larger relays need to adjust their settings.
+
+### Features
+
+- Reduce maximal_queue_lifetime from 5d to 2d
+- Disable negative cache in unbound (#992)
+- *(mtail)* Add incoming_mailer_daemon_mail_count
+- *(postfix)* Disable processing of MIME headers
+- *(dovecot)* Advertise privacy_mail as admin contact, drop server comment
+
+### Bug Fixes
+
+- Set relay restrictions per smtpd service with default reject
+- Reduce maxproc for filtermail-transport LMTP client to 500
+- Core 2.50.0 does not have delete_server_after config anymore.
+- Check if all required ports are available for filtermail (#983)
+- Always deploy unbound.conf.d/chatmail.conf (#993)
+- Expire empty directories (#994)
+- Crypt-r dependency was declared for wrong Python version
+- Always overwrite /etc/resolv.conf, even if it is a symbolic link
+- Pass kwargs to files.put()
+- List Iroh proxy endpoints used by 0.35 and 1.0, drop stale /relay/probe from earlier versions
+- Fix port discovery when ss -tulpn shows dovecot before stats
+
+### Documentation
+
+- Add scripts/initenv.sh to upgrade instructions
+- Update overview diagrams (#995)
+- *(overview)* Remove mermaid styles from 'Accepting and delivering mail' (#1009)
+- *(README.md)* Clarify security enforcement (#1011)
+
+### Miscellaneous Tasks
+
+- *(ci)* Auto-trigger docker build on release tag push
+- *(acmetool)* Update let's encrypt ToS link to 1.8
+- *(ci)* Update doc staging upload path
+- *(ci)* Fix docs upload path
+
+### Refactor
+
+- *(postfix)* Remove unused "filter" lmtp service
+- Install dns-root-data instead of using unbound-anchor
+- *(deps)* Remove domain-validator dependency
+
+### Testing
+
+- Set socket security for IMAP and SMTP to "TLS" in "dclogin"
+
 ## [1.11.0] - 2026-05-15
 
 ### Breaking Changes

RELEASE.md

@@ -1,15 +1,13 @@
 # Releasing a new version of chatmail relay
 
-For example, to release version 1.9.0 of chatmail relay, do the following steps.
+For example, to release version 1.13.0 of chatmail relay, do the following steps.
 
-1. Update the changelog: `git cliff --unreleased --tag 1.9.0 --prepend CHANGELOG.md` or `git cliff -u -t 1.9.0 -p CHANGELOG.md`.
+1. Update the changelog: `git cliff --unreleased --tag 1.13.0 --prepend CHANGELOG.md` or `git cliff -u -t 1.13.0 -p CHANGELOG.md`.
 
 2. Open the changelog in the editor, edit it if required.
 
 3. Commit the changes to the changelog with a commit message `chore(release): prepare for 1.9.0`.
 
-3. Tag the release: `git tag --annotate 1.9.0`.
+4. Open a PR with the new commit, merge it to main after review.
 
-4. Push the release tag: `git push origin 1.9.0`.
-
-5. Create a GitHub release: `gh release create 1.9.0`.
+5. In the web interface, create a GitHub release, tell it to create a new tag.

chatmaild/pyproject.toml

@@ -8,6 +8,7 @@ version = "0.3"
 dependencies = [
   "iniconfig",
   "filelock",
+  "psutil",
   "requests",
   "crypt-r >= 3.13.1 ; python_version >= '3.13'",
 ]

chatmaild/src/chatmaild/config.py

@@ -75,6 +75,16 @@ class Config:
         self.privacy_pdo = params.pop("privacy_pdo", None)
         self.privacy_supervisor = params.pop("privacy_supervisor", None)
 
+        self.max_load_1m = float(params.pop("max_load_1m", 5))
+        self.min_available_memory_mb = parse_size_mb(
+            params.pop("min_available_memory", "200M")
+        )
+        self.min_free_disk_space_mb = parse_size_mb(
+            params.pop("min_free_disk_space", "1G")
+        )
+        self.max_imap_connections = int(params.pop("max_imap_connections", 10000))
+        self.max_smtp_connections = int(params.pop("max_smtp_connections", 1000))
+
         # TLS certificate management.
         # If tls_external_cert_and_key is set, use externally managed certs.
         # Otherwise derived from the domain name:

chatmaild/src/chatmaild/doveauth.py

@@ -14,6 +14,7 @@ except ImportError:
 from .config import Config, read_config
 from .dictproxy import DictProxy
 from .migrate_db import migrate_from_db_to_maildir
+from .syslimits import has_sufficient_resources
 
 NOCREATE_FILE = "/etc/chatmail-nocreate"
 VALID_LOCALPART_RE = re.compile(r"^[a-z0-9._-]+$")
@@ -147,6 +148,8 @@ class AuthDictProxy(DictProxy):
             return userdata
         if not is_allowed_to_create(self.config, addr, cleartext_password):
             return
+        if not has_sufficient_resources(self.config):
+            return
 
         lock = filelock.FileLock(str(user.password_path) + ".lock", timeout=5)
         with lock:

chatmaild/src/chatmaild/ini/chatmail.ini.f

@@ -42,6 +42,33 @@ mail_domain = {mail_domain}
 # minimum length a password must have
 #password_min_length = 9
 
+#
+# System resource limits
+#
+
+# The following three limits refuse creation of new addresses
+# while existing addresses keep working.
+# Rejections are logged by the doveauth service.
+
+# Maximum 1-minute load average, as reported by "uptime";
+# it counts processes waiting for disk I/O as well as for CPU.
+#max_load_1m = 5
+
+# Minimum memory available without swapping.
+#min_available_memory = 200M
+
+# Minimum free disk space on the file system holding the mailboxes.
+#min_free_disk_space = 1G
+
+# Maximum number of concurrent IMAP connections
+# (the Dovecot imap process limit).
+#max_imap_connections = 10000
+
+# Maximum number of concurrent SMTP connections
+# on each of the submission and smtps ports (the Postfix process limit).
+# A single client IP may use up to a fifth of this.
+#max_smtp_connections = 1000
+
 # Use externally managed TLS certificates instead of built-in acmetool.
 # Paths refer to files on the deployment server (not the build machine).
 # Both files must already exist before running cmdeploy.

chatmaild/src/chatmaild/syslimits.py

@@ -0,0 +1,32 @@
+"""Detect whether the system is at its limits."""
+
+import logging
+
+import psutil
+
+MB = 1024 * 1024
+
+
+def read_value(getter):
+    try:
+        return getter()
+    except Exception as e:
+        logging.warning("ignoring unreadable system limit: %s", e)
+        return None
+
+
+def has_sufficient_resources(config):
+    """Return False if load, memory or disk exceeds a configured limit."""
+    load = read_value(lambda: psutil.getloadavg()[0])
+    mem = read_value(lambda: psutil.virtual_memory().available // MB)
+    disk = read_value(lambda: psutil.disk_usage(str(config.mailboxes_dir)).free // MB)
+    if load is not None and load > config.max_load_1m:
+        msg = f"load avg {load:.2f} > {config.max_load_1m:.2f}"
+    elif mem is not None and mem < config.min_available_memory_mb:
+        msg = f"available memory {mem}MB < {config.min_available_memory_mb}MB"
+    elif disk is not None and disk < config.min_free_disk_space_mb:
+        msg = f"free disk {disk}MB < {config.min_free_disk_space_mb}MB"
+    else:
+        return True
+    logging.warning("registration rejected: %s", msg)
+    return False

chatmaild/src/chatmaild/tests/plugin.py

@@ -20,6 +20,10 @@ def make_config(tmp_path):
         basedir.mkdir(parents=True, exist_ok=True)
         overrides = settings.copy() if settings else {}
         overrides["mailboxes_dir"] = str(basedir)
+        # permissive resource limits so tests never depend on host load/memory/disk
+        overrides.setdefault("max_load_1m", "99999")
+        overrides.setdefault("min_available_memory", "0")
+        overrides.setdefault("min_free_disk_space", "0")
         write_initial_config(inipath, mail_domain, overrides=overrides)
         return read_config(inipath)
 

chatmaild/src/chatmaild/tests/test_config.py

@@ -45,6 +45,8 @@ def test_read_config_basic_using_defaults(tmp_path, maildomain):
     assert example_config.username_min_length == 9
     assert example_config.username_max_length == 9
     assert example_config.password_min_length == 9
+    assert example_config.max_imap_connections == 10000
+    assert example_config.max_smtp_connections == 1000
     assert example_config._unused_keys == []
 
 

chatmaild/src/chatmaild/tests/test_doveauth.py

@@ -202,3 +202,17 @@ def test_50_concurrent_lookups_different_accounts(gencreds, dictproxy):
         res = results.get()
         if res is not None:
             pytest.fail(f"concurrent lookup failed\n{res}")
+
+
+def test_insufficient_resources_block_creation_not_existing_logins(
+    dictproxy, gencreds, monkeypatch
+):
+    addr, password = gencreds()
+    assert dictproxy.lookup_passdb(addr, password)
+
+    monkeypatch.setattr(
+        chatmaild.doveauth, "has_sufficient_resources", lambda config: False
+    )
+    newaddr, newpassword = gencreds()
+    assert not dictproxy.lookup_passdb(newaddr, newpassword)
+    assert dictproxy.lookup_passdb(addr, password)

chatmaild/src/chatmaild/tests/test_syslimits.py

@@ -0,0 +1,45 @@
+import shutil
+
+import psutil
+
+from chatmaild.syslimits import has_sufficient_resources
+
+PERMISSIVE = {
+    "max_load_1m": "99999",
+    "min_available_memory": "0",
+    "min_free_disk_space": "0",
+}
+
+
+def test_rejects_constrained_system(make_config, caplog):
+    assert has_sufficient_resources(make_config("chat.example.org", PERMISSIVE))
+    for settings in (
+        {"max_load_1m": "-1.0"},
+        {"min_available_memory": "99999999G"},
+        {"min_free_disk_space": "99999999G"},
+    ):
+        config = make_config("chat.example.org", PERMISSIVE | settings)
+        caplog.clear()
+        assert not has_sufficient_resources(config), settings
+        assert "registration rejected" in caplog.text
+
+
+def test_unreadable_disk_does_not_reject(make_config, caplog):
+    config = make_config(
+        "chat.example.org", PERMISSIVE | {"min_free_disk_space": "99999999G"}
+    )
+    shutil.rmtree(config.mailboxes_dir)
+    assert has_sufficient_resources(config)
+    assert "ignoring" in caplog.text
+
+
+def test_one_unreadable_value_keeps_other_checks(make_config, monkeypatch, caplog):
+    def raise_error(*args):
+        raise psutil.Error("dud")
+
+    monkeypatch.setattr(psutil, "getloadavg", raise_error)
+    config = make_config(
+        "chat.example.org", PERMISSIVE | {"min_free_disk_space": "99999999G"}
+    )
+    assert not has_sufficient_resources(config)
+    assert "ignoring" in caplog.text

cmdeploy/src/cmdeploy/acmetool/response-file.yaml.j2

@@ -1,2 +1,2 @@
 "acme-enter-email": "{{ email }}"
-"acme-agreement:https://letsencrypt.org/documents/LE-SA-v1.7-June-04-2026.pdf": true
+"acme-agreement:https://letsencrypt.org/documents/LE-SA-v1.8-July-06-2026.pdf": true

cmdeploy/src/cmdeploy/deployers.py

@@ -98,6 +98,23 @@ def _install_remote_venv_with_chatmaild(deployer) -> None:
         dest=remote_dist_file,
     )
 
+    # Remove venv if its Python major.minor doesn't match the system Python
+    server.shell(
+        name="remove stale chatmaild venv if python version changed",
+        commands=[
+            "\n".join(
+                [
+                    r"re='[0-9]+\.[0-9]+'",  # major.minor out of 'Python X.Y.Z'
+                    'sys_version=$(python3 --version 2>/dev/null | grep -oE "$re")',
+                    f'venv_version=$({remote_venv_dir}/bin/python --version 2>/dev/null | grep -oE "$re")',
+                    # an empty sys_version means we could not tell: keep the venv
+                    f'[ -z "$sys_version" ] || [ "$sys_version" = "$venv_version" ] '
+                    f"|| rm -rf {remote_venv_dir}",
+                ]
+            )
+        ],
+    )
+
     pip.virtualenv(
         name=f"chatmaild virtualenv {remote_venv_dir}",
         path=remote_venv_dir,
@@ -138,9 +155,22 @@ class UnboundDeployer(Deployer):
         # On an IPv4-only system, if unbound is started but not configured,
         # it causes subsequent steps to fail to resolve hosts.
         with blocked_service_startup():
+            # dns-root-data is an optional package
+            # that contains /usr/share/dns/root.key
+            #
+            # This file is copied into /var/lib/unbound/root.key
+            # at the start of "unbound" systemd unit
+            # by /usr/libexec/unbound-helper shell script
+            # from the "unbound" package as of version 1.17.1-2+deb12u4
+            #
+            # The same /var/lib/unbound/root.key can be retrieved directly
+            # following the procedure from
+            # <https://www.rfc-editor.org/info/rfc7958/#section-3.1>
+            # with "unbound-anchor -a /var/lib/unbound/root.key"
+            # We don't install and use "unbound-anchor".
             apt.packages(
                 name="Install unbound",
-                packages=["unbound", "unbound-anchor", "dnsutils"],
+                packages=["unbound", "dns-root-data", "dnsutils"],
             )
 
     def configure(self):
@@ -166,12 +196,6 @@ class UnboundDeployer(Deployer):
             dest="/etc/resolv.conf",
             force=True,
         )
-        server.shell(
-            name="Generate root keys for validating DNSSEC",
-            commands=[
-                "unbound-anchor -a /var/lib/unbound/root.key || true",
-            ],
-        )
         self.ensure_directory(
             path="/etc/unbound/unbound.conf.d",
         )
@@ -246,6 +270,13 @@ class LegacyRemoveDeployer(Deployer):
     def install(self):
         apt.packages(name="Remove rspamd", packages="rspamd", present=False)
 
+        # unbound-anchor was used to download /var/lib/unbound/root.key
+        # It is replaced by dns-root-data which contains /usr/share/dns/root.key.
+        # unbound systemd unit copies /usr/share/dns/root.key
+        # into /var/lib/unbound/root.key automatically on start
+        # as long as /usr/share/dns/root.key is present.
+        apt.packages(name="Remove unbound-anchor", packages="unbound-anchor", present=False)
+
         # remove historic expunge script
         # which is now implemented through a systemd timer (chatmail-expire)
         self.remove_file("/etc/cron.d/expunge")
@@ -356,6 +387,8 @@ class ChatmailVenvDeployer(Deployer):
     def __init__(self, config):
         self.config = config
         self.units = (
+            # doveauth must restart when chatmaild/ini file changes
+            "doveauth",
             "chatmail-metadata",
             "lastlogin",
             "chatmail-expire",
@@ -390,6 +423,12 @@ class ChatmailDeployer(Deployer):
             src=BytesIO(b'APT::Install-Recommends "false";\n'),
             dest="/etc/apt/apt.conf.d/00InstallRecommends",
         )
+        # Pin dovecot-* to priority -1 before any apt operation, apt should
+        # never manage dovecot as our version might be lower than the distro's.
+        self.put_file(
+            src=StringIO("Package: dovecot-*\nPin: version *\nPin-Priority: -1\n"),
+            dest="/etc/apt/preferences.d/pin-dovecot",
+        )
         apt.update(name="apt update", cache_time=24 * 3600)
         apt.upgrade(name="upgrade apt packages", auto_remove=True)
 
@@ -504,10 +543,10 @@ def deploy_chatmail(config_path: Path, disable_mail: bool, website_only: bool) -
             ("nginx", 443),
             (["master", "smtpd"], 465),
             (["master", "smtpd"], 587),
-            (["imap-login", "dovecot"], 993),
+            (["dovecot", "imap-login"], 993),
             ("iroh-relay", 3340),
             ("mtail", 3903),
-            ("stats", 3904),
+            (["dovecot", "stats"], 3904),
             ("nginx", 8443),
             (["master", "smtpd"], config.postfix_reinject_port),
             (["master", "smtpd"], config.postfix_reinject_port_incoming),

cmdeploy/src/cmdeploy/dovecot/deployer.py

@@ -1,11 +1,10 @@
-import io
 import urllib.request
 
 from chatmaild.config import Config
 from pyinfra import host
 from pyinfra.facts.deb import DebPackages
 from pyinfra.facts.server import Arch, Command, Sysctl
-from pyinfra.operations import apt, files, server
+from pyinfra.operations import files, server
 
 from cmdeploy.basedeploy import (
     Deployer,
@@ -15,16 +14,31 @@ from cmdeploy.basedeploy import (
     is_in_container,
 )
 
-DOVECOT_ARCHIVE_VERSION = "2.3.21+dfsg1-3"
-DOVECOT_PACKAGE_VERSION = f"1:{DOVECOT_ARCHIVE_VERSION}"
+# distro-neutral base version, as committed in chatmail/dovecot debian/changelog
+DOVECOT_ARCHIVE_VERSION = "2.3.21+dfsg1-3+chatmail2"
+
+VERSION_ID_CMD = "grep '^VERSION_ID=' /etc/os-release"
+
+
+def _stamped_version(deb_release: int) -> str:
+    """Version as built, including the per-distro suffix stamped by
+    chatmail/dovecot CI into package version and filename."""
+    return f"{DOVECOT_ARCHIVE_VERSION}+deb{deb_release}u1"
+
 
 DOVECOT_SHA256 = {
-    ("core", "amd64"): "dd060706f52a306fa863d874717210b9fe10536c824afe1790eec247ded5b27d",
-    ("core", "arm64"): "e7548e8a82929722e973629ecc40fcfa886894cef3db88f23535149e7f730dc9",
-    ("imapd", "amd64"): "8d8dc6fc00bbb6cdb25d345844f41ce2f1c53f764b79a838eb2a03103eebfa86",
-    ("imapd", "arm64"): "178fa877ddd5df9930e8308b518f4b07df10e759050725f8217a0c1fb3fd707f",
-    ("lmtpd", "amd64"): "2f69ba5e35363de50962d42cccbfe4ed8495265044e244007d7ccddad77513ab",
-    ("lmtpd", "arm64"): "89f52fb36524f5877a177dff4a713ba771fd3f91f22ed0af7238d495e143b38f",
+    ("amd64", 12, "core"): "ac3977264d9b9a6fcec53fd3f5cdd2a79ca8aa0324de530c07e535008540826e",
+    ("arm64", 12, "core"): "21626c9c9b52cbdcf1a17b5c09e3c4043e69aa371bf83cc2fcb3b7ddaecdc109",
+    ("amd64", 13, "core"): "47c242ef23c17e700ac19d52d82c9fdb2ebd757d8beb3a7f6781d2de59f87bd0",
+    ("arm64", 13, "core"): "c14c53f112c875f698c4cb6e5870c605cd0a9dd98d35a66e94ceb1827f8020a3",
+    ("amd64", 12, "imapd"): "92a7ab5fc7dc32886a0c34404f919f1335d397b48c467e0c1ef77e56978f60ea",
+    ("arm64", 12, "imapd"): "9369fd566fec4df109ef23debf34ea0417ae85beb29cbe7de619d4d1f31b120c",
+    ("amd64", 13, "imapd"): "e38cc1266455f937ed62f971ea859c47e1a99247841ed0ad946963b524cfdbc5",
+    ("arm64", 13, "imapd"): "11d97dabf23171b37f8b1335dfdb81d408f8b95391aea6d4066aecc9fde01dfe",
+    ("amd64", 12, "lmtpd"): "dc3de473789969f7dd3504ac8783da5e42a446d2d7a305a4e9d7081a6dfe71ab",
+    ("arm64", 12, "lmtpd"): "ae2cbd6c5c43f6d8e2172997b055448f4c79238e2f99cd9ab9200a7d9f548908",
+    ("amd64", 13, "lmtpd"): "833b243e28c7baff141ecf37456e310f5d836e7944a3b9f2fe5074adf0d6a418",
+    ("arm64", 13, "lmtpd"): "55af47a121ba7e23966b20ddaab2dff7feba4b34677864e045e31a702afa180d",
 }
 
 
@@ -34,38 +48,34 @@ class DovecotDeployer(Deployer):
     def __init__(self, config, disable_mail):
         self.config = config
         self.disable_mail = disable_mail
-        self.units = ["doveauth"]
+        self.units = []
 
     def install(self):
         arch = host.get_fact(Arch)
+        deb_release = _parse_version_id(host.get_fact(Command, VERSION_ID_CMD))
         with blocked_service_startup():
             debs = []
             for pkg in ("core", "imapd", "lmtpd"):
-                deb, changed = _download_dovecot_package(pkg, arch)
+                deb, changed = _download_dovecot_package(pkg, arch, deb_release)
                 self.need_restart |= changed
                 if deb:
                     debs.append(deb)
             if debs:
                 deb_list = " ".join(debs)
-                # First dpkg may fail on missing dependencies (stderr suppressed);
-                # apt-get --fix-broken pulls them in, then dpkg retries cleanly.
+                # apt-get install with local .deb paths resolves depends
+                # against the configured repos (e.g. pulls libwrap0),
+                # The pin file written earlier by ChatmailDeployer prevents apt
+                # from installing a 'wrong' version
                 server.shell(
                     name="Install dovecot packages",
                     commands=[
-                        f"dpkg --force-confdef --force-confold -i {deb_list} 2> /dev/null || true",
-                        "DEBIAN_FRONTEND=noninteractive apt-get -y --fix-broken install",
-                        f"dpkg --force-confdef --force-confold -i {deb_list}",
+                        "DEBIAN_FRONTEND=noninteractive apt-get install -y "
+                        '-o Dpkg::Options::="--force-confdef" '
+                        '-o Dpkg::Options::="--force-confold" '
+                        f"--allow-downgrades {deb_list}",
                     ],
                 )
                 self.need_restart = True
-        self.put_file(
-            src=io.StringIO(
-                "Package: dovecot-*\n"
-                "Pin: version *\n"
-                "Pin-Priority: -1\n"
-            ),
-            dest="/etc/apt/preferences.d/pin-dovecot",
-        )
 
     def configure(self):
         configure_remote_units(self, self.config.mail_domain_bare, self.units)
@@ -78,7 +88,7 @@ class DovecotDeployer(Deployer):
         if not self.disable_mail and not self.need_restart:
             stale = host.get_fact(
                 Command,
-                'pid=$(systemctl show -p MainPID --value dovecot.service 2>/dev/null);'
+                "pid=$(systemctl show -p MainPID --value dovecot.service 2>/dev/null);"
                 ' [ "${pid:-0}" != "0" ] && readlink "/proc/$pid/exe" 2>/dev/null | grep -q "(deleted)"'
                 " && echo STALE || true",
             )
@@ -93,6 +103,15 @@ class DovecotDeployer(Deployer):
         )
 
 
+def _parse_version_id(version_line: str) -> int:
+    """Debian major release from an /etc/os-release VERSION_ID line."""
+    _, _, raw = (version_line or "").strip().partition("=")
+    try:
+        return int(raw.strip('"'))
+    except ValueError:
+        raise ValueError(f"cannot determine Debian release from {version_line!r}")
+
+
 def _pick_url(primary, fallback):
     try:
         req = urllib.request.Request(primary, method="HEAD")
@@ -102,27 +121,36 @@ def _pick_url(primary, fallback):
         return fallback
 
 
-def _download_dovecot_package(package: str, arch: str) -> tuple[str | None, bool]:
+def _download_dovecot_package(package: str, arch: str, deb_release: int) -> tuple[str | None, bool]:
     """Download a dovecot .deb if needed, return (path, changed)."""
     arch = "amd64" if arch == "x86_64" else arch
     arch = "arm64" if arch == "aarch64" else arch
 
     pkg_name = f"dovecot-{package}"
-    sha256 = DOVECOT_SHA256.get((package, arch))
-    if sha256 is None:
-        op = apt.packages(packages=[pkg_name])
-        return None, bool(getattr(op, "changed", False))
+    try:
+        # never fall back to the distro package: it is pinned to -1 and would
+        # in any case be a version we did not build and do not support
+        sha256 = DOVECOT_SHA256[(arch, deb_release, package)]
+    except KeyError:
+        raise ValueError(f"no dovecot build for {pkg_name} on deb{deb_release}/{arch}")
 
+    stamped_version = _stamped_version(deb_release)
     installed_versions = host.get_fact(DebPackages).get(pkg_name, [])
-    if DOVECOT_PACKAGE_VERSION in installed_versions:
+    if f"1:{stamped_version}" in installed_versions:
         return None, False
 
-    url_version = DOVECOT_ARCHIVE_VERSION.replace("+", "%2B")
-    deb_base = f"{pkg_name}_{url_version}_{arch}.deb"
-    primary_url = f"https://download.delta.chat/dovecot/{deb_base}"
-    fallback_url = f"https://github.com/chatmail/dovecot/releases/download/upstream%2F{url_version}/{deb_base}"
+    # Primary URL: flat structure with distro suffix in filename
+    primary_deb = f"{pkg_name}_{stamped_version}_{arch}.deb"
+    primary_url = f"https://download.delta.chat/dovecot/{primary_deb}"
+    # GitHub release files: escaped + in filename; the release tag stays
+    # distro-neutral, both distros ship in one combined release
+    tag_version = DOVECOT_ARCHIVE_VERSION.replace("+", "%2B")
+    fallback_deb = f"{pkg_name}_{stamped_version.replace('+', '%2B')}_{arch}.deb"
+    fallback_url = (
+        f"https://github.com/chatmail/dovecot/releases/download/upstream%2F{tag_version}/{fallback_deb}"
+    )
     url = _pick_url(primary_url, fallback_url)
-    deb_filename = f"/root/{deb_base}"
+    deb_filename = f"/root/{primary_deb}"
 
     files.download(
         name=f"Download {pkg_name}",
@@ -134,6 +162,7 @@ def _download_dovecot_package(package: str, arch: str) -> tuple[str | None, bool
 
     return deb_filename, True
 
+
 def _configure_dovecot(deployer, config: Config, debug: bool = False):
     """Configures Dovecot IMAP server."""
     deployer.put_template(
@@ -144,9 +173,7 @@ def _configure_dovecot(deployer, config: Config, debug: bool = False):
         disable_ipv6=config.disable_ipv6,
     )
     deployer.put_file("dovecot/auth.conf", "/etc/dovecot/auth.conf")
-    deployer.put_file(
-        "dovecot/push_notification.lua", "/etc/dovecot/push_notification.lua"
-    )
+    deployer.put_file("dovecot/push_notification.lua", "/etc/dovecot/push_notification.lua")
 
     # as per https://doc.dovecot.org/2.3/configuration_manual/os/
     # it is recommended to set the following inotify limits

cmdeploy/src/cmdeploy/dovecot/dovecot.conf.j2

@@ -37,11 +37,15 @@ default_client_limit = 20000
 # the following warning will be logged:
 #   Warning: service(imap): process_limit (1024) reached, client connections are being dropped
 service imap {
-  process_limit = 50000
+  process_limit = {{ config.max_imap_connections }}
 }
 
-mail_server_admin = mailto:root@{{ config.mail_domain }}
-mail_server_comment = Chatmail server
+{% if config.privacy_mail %}
+# Advertised to clients as IMAP METADATA /shared/admin (RFC 5464).
+# The privacy_mail contact from chatmail.ini is used because it is
+# the only address set by an operator.
+mail_server_admin = mailto:{{ config.privacy_mail }}
+{% endif %}
 
 # `zlib` enables compressing messages stored in the maildir.
 # See

cmdeploy/src/cmdeploy/filtermail/deployer.py

@@ -20,10 +20,10 @@ class FiltermailDeployer(Deployer):
             return
 
         arch = host.get_fact(facts.server.Arch)
-        url = f"https://github.com/chatmail/filtermail/releases/download/v0.7.1/filtermail-{arch}"
+        url = f"https://github.com/chatmail/filtermail/releases/download/v0.7.4/filtermail-{arch}"
         sha256sum = {
-            "x86_64": "fc2d8141166f8561b9711fb68c5327fc9421f814c46dc69671a4605a95b175c0",
-            "aarch64": "37e52c5ddb373ef29b5ead89658407c53f48d10ce055a2dbd9c606fa1ebd5f7f",
+            "x86_64": "484cb8dff083134aefba9fce4a6b7ef4784a0f0e28e5108ecf8bb9e58a44fd2c",
+            "aarch64": "66aa0ca2ca9add7a12d92883d76f8786384092adfde24a3d3a1d0b1f30d23a9e",
         }[arch]
         self.download_executable(url, self.bin_path, sha256sum)
 

cmdeploy/src/cmdeploy/mtail/delivered_mail.mtail

@@ -28,6 +28,13 @@ counter created_nonci_accounts
   }
 }
 
+# doveauth refusing new addresses because a chatmail.ini
+# system resource limit is exceeded.
+counter rejected_registrations
+/registration rejected: / {
+  rejected_registrations++
+}
+
 counter postfix_timeouts
 /timeout after DATA/ {
   postfix_timeouts++
@@ -73,6 +80,11 @@ counter incoming_unencrypted_mail_count
   filtered_incoming_mail_count++
 }
 
+counter incoming_mailer_daemon_mail_count
+/Incoming: Filtering mailer-daemon message from/ {
+  incoming_mailer_daemon_mail_count++
+  filtered_incoming_mail_count++
+}
 
 counter rejected_unencrypted_mail_count
 /Rejected unencrypted mail/ {

cmdeploy/src/cmdeploy/nginx/nginx.conf.j2

@@ -128,7 +128,10 @@ http {
                         proxy_set_header Connection "upgrade";
                 }
 
-		location /relay/probe {
+		# Endpoints Iroh uses for net_report probes and that clients
+		# probe to tell whether this relay works. Both paths are served
+		# by iroh-relay 0.35 and by the 1.0 line.
+		location /ping {
                         proxy_pass http://127.0.0.1:3340;
                         proxy_http_version 1.1;
 		}

cmdeploy/src/cmdeploy/postfix/main.cf.j2

@@ -85,6 +85,14 @@ inet_protocols = all
 
 lmtp_header_checks = regexp:/etc/postfix/lmtp_header_cleanup
 
+# Do not apply header checks to MIME headers
+# and other headers that are actually part of the message body.
+# Reference:
+# <https://www.postfix.org/postconf.5.html#disable_mime_input_processing>
+# <https://www.postfix.org/header_checks.5.html>
+# <https://stump.io/blog/2020/11/29/a-little-gotcha-with-postfixs-header_checks/>
+disable_mime_input_processing = yes
+
 mua_client_restrictions = permit_sasl_authenticated, reject
 mua_sender_restrictions = reject_sender_login_mismatch, permit_sasl_authenticated, reject
 mua_helo_restrictions = permit_mynetworks, reject_invalid_helo_hostname, reject_non_fqdn_helo_hostname, permit

cmdeploy/src/cmdeploy/postfix/master.cf.j2

@@ -18,7 +18,7 @@ smtp      inet  n       -       y       -       -       smtpd
   -o smtpd_tls_mandatory_protocols=>=TLSv1.2
   -o smtpd_proxy_filter=127.0.0.1:{{ config.filtermail_smtp_port_incoming }}
   -o smtpd_relay_restrictions=reject_unauth_destination
-submission inet n       -       y       -       5000    smtpd
+submission inet n       -       y       -       {{ config.max_smtp_connections }}    smtpd
   -o syslog_name=postfix/submission
   -o smtpd_tls_security_level=encrypt
   -o smtpd_tls_mandatory_protocols=>=TLSv1.3
@@ -32,9 +32,9 @@ submission inet n       -       y       -       5000    smtpd
   -o smtpd_sender_restrictions=$mua_sender_restrictions
   -o smtpd_recipient_restrictions=
   -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
-  -o smtpd_client_connection_count_limit=1000
+  -o smtpd_client_connection_count_limit={{ config.max_smtp_connections // 5 }}
   -o smtpd_proxy_filter=127.0.0.1:{{ config.filtermail_smtp_port }}
-smtps     inet  n       -       y       -       5000    smtpd
+smtps     inet  n       -       y       -       {{ config.max_smtp_connections }}    smtpd
   -o syslog_name=postfix/smtps
   -o smtpd_tls_wrappermode=yes
   -o smtpd_tls_security_level=encrypt
@@ -48,7 +48,7 @@ smtps     inet  n       -       y       -       5000    smtpd
   -o smtpd_sender_restrictions=$mua_sender_restrictions
   -o smtpd_recipient_restrictions=
   -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
-  -o smtpd_client_connection_count_limit=1000
+  -o smtpd_client_connection_count_limit={{ config.max_smtp_connections // 5 }}
   -o smtpd_proxy_filter=127.0.0.1:{{ config.filtermail_smtp_port }}
 #628       inet  n       -       y       -       -       qmqpd
 pickup    unix  n       -       y       60      1       pickup
@@ -76,7 +76,6 @@ lmtp      unix  -       -       y       -       -       lmtp
 anvil     unix  -       -       y       -       1       anvil
 scache    unix  -       -       y       -       1       scache
 postlog   unix-dgram n  -       n       -       1       postlogd
-filter    unix -        n       n       -       -       lmtp
 # Local SMTP server for reinjecting outgoing filtered mail.
 127.0.0.1:{{ config.postfix_reinject_port }} inet  n       -       n       -       100      smtpd
   -o syslog_name=postfix/reinject

cmdeploy/src/cmdeploy/tests/test_dovecot_deployer.py

@@ -3,29 +3,40 @@ from types import SimpleNamespace
 
 import pytest
 from pyinfra.facts.deb import DebPackages
+from pyinfra.facts.server import Command
 
 from cmdeploy.dovecot import deployer as dovecot_deployer
 
 
+def _fact_name(key):
+    if isinstance(key, tuple):
+        return f"{key[0].__name__}{key[1:]!r}"
+    return key.__name__
+
+
 def make_host(*fact_pairs):
-    """Build a mock host; get_fact(cls) dispatches to the provided facts mapping.
+    """Build a mock host; get_fact() dispatches to the provided facts mapping.
 
     Args:
-        *fact_pairs: tuples of (fact_class, fact_value) to register
+        *fact_pairs: (fact_class, value) to match any call of that fact, or
+            ((fact_class, *args), value) to match one specific call. Needed
+            for Command, which install() and check_restart() invoke with
+            different scripts; a bare Command entry would serve both.
 
     Returns:
         SimpleNamespace with get_fact that raises a clear error if an
-        unexpected fact type is requested.
+        unregistered fact is requested.
     """
     facts = dict(fact_pairs)
 
-    def get_fact(cls):
-        if cls not in facts:
-            registered = ", ".join(c.__name__ for c in facts)
-            raise LookupError(
-                f"unexpected get_fact({cls.__name__}); only registered: {registered}"
-            )
-        return facts[cls]
+    def get_fact(cls, *args):
+        for key in ((cls, *args), cls):
+            if key in facts:
+                return facts[key]
+        registered = ", ".join(_fact_name(k) for k in facts)
+        raise LookupError(
+            f"unexpected get_fact({_fact_name((cls, *args))}); only registered: {registered}"
+        )
 
     return SimpleNamespace(get_fact=get_fact)
 
@@ -64,7 +75,9 @@ def track_shell(monkeypatch):
 
 
 def test_download_dovecot_package_skips_epoch_matched_install(monkeypatch):
-    epoch_version = dovecot_deployer.DOVECOT_PACKAGE_VERSION
+    # what dpkg reports after installing our deb: epoch + the +debNu1 suffix
+    # that chatmail/dovecot CI stamps via dch before building
+    epoch_version = f"1:{dovecot_deployer._stamped_version(12)}"
     downloads = []
     monkeypatch.setattr(
         dovecot_deployer,
@@ -82,15 +95,17 @@ def test_download_dovecot_package_skips_epoch_matched_install(monkeypatch):
         lambda **kwargs: downloads.append(kwargs),
     )
 
-    deb, changed = dovecot_deployer._download_dovecot_package("core", "amd64")
+    deb, changed = dovecot_deployer._download_dovecot_package("core", "amd64", deb_release=12)
 
     assert deb is None, f"expected no deb path when version matches, got {deb!r}"
     assert changed is False, "should not flag changed when version already installed"
     assert downloads == [], "should not download when version already installed"
 
 
+@pytest.mark.parametrize("deb_release", [12, 13])
+@pytest.mark.parametrize("arch", ["amd64", "arm64"])
 def test_download_dovecot_package_uses_archive_version_for_url_and_filename(
-    monkeypatch,
+    monkeypatch, deb_release, arch
 ):
     downloads = []
     monkeypatch.setattr(
@@ -109,18 +124,26 @@ def test_download_dovecot_package_uses_archive_version_for_url_and_filename(
         lambda **kwargs: downloads.append(kwargs),
     )
 
-    deb, changed = dovecot_deployer._download_dovecot_package("core", "amd64")
+    deb, changed = dovecot_deployer._download_dovecot_package(
+        "core", arch, deb_release=deb_release
+    )
 
-    archive_version = dovecot_deployer.DOVECOT_ARCHIVE_VERSION.replace("+", "%2B")
-    expected_deb = f"/root/dovecot-core_{archive_version}_amd64.deb"
+    stamped = dovecot_deployer._stamped_version(deb_release)
+    expected_deb = f"/root/dovecot-core_{stamped}_{arch}.deb"
 
-    # Verify the returned path uses archive version, not package version (with epoch)
+    # path uses the stamped version, and deb filenames never carry the epoch
     assert changed is True, "should flag changed when package not yet installed"
     assert deb == expected_deb, f"deb path mismatch: {deb!r} != {expected_deb!r}"
-    assert dovecot_deployer.DOVECOT_PACKAGE_VERSION not in deb, (
-        f"deb path should use archive version (no epoch), got {deb!r}"
-    )
+    assert "1:" not in deb, f"deb filename must not contain the epoch, got {deb!r}"
     assert len(downloads) == 1, "files.download should be called exactly once"
+    # the checksum is the security boundary: verify the right table row is used
+    assert (
+        downloads[0]["sha256sum"]
+        == dovecot_deployer.DOVECOT_SHA256[(arch, deb_release, "core")]
+    ), "must pass the sha256 matching (arch, release, package)"
+    assert f"deb{deb_release}u1" in downloads[0]["src"], (
+        f"download URL should carry the deb{deb_release} suffix, got {downloads[0]['src']!r}"
+    )
 
 
 def test_install_skips_dpkg_path_when_epoch_matched_packages_present(
@@ -133,12 +156,13 @@ def test_install_skips_dpkg_path_when_epoch_matched_packages_present(
             (
                 dovecot_deployer.DebPackages,
                 {
-                    "dovecot-core": [dovecot_deployer.DOVECOT_PACKAGE_VERSION],
-                    "dovecot-imapd": [dovecot_deployer.DOVECOT_PACKAGE_VERSION],
-                    "dovecot-lmtpd": [dovecot_deployer.DOVECOT_PACKAGE_VERSION],
+                    "dovecot-core": [f"1:{dovecot_deployer._stamped_version(12)}"],
+                    "dovecot-imapd": [f"1:{dovecot_deployer._stamped_version(12)}"],
+                    "dovecot-lmtpd": [f"1:{dovecot_deployer._stamped_version(12)}"],
                 },
             ),
             (dovecot_deployer.Arch, "x86_64"),
+            ((Command, dovecot_deployer.VERSION_ID_CMD), 'VERSION_ID="12"'),
         ),
     )
     downloads = []
@@ -152,41 +176,26 @@ def test_install_skips_dpkg_path_when_epoch_matched_packages_present(
 
     assert downloads == [], "should not download when all packages epoch-matched"
     assert track_shell == [], "should not run dpkg when all packages epoch-matched"
-    assert deployer.need_restart is False, (
-        "need_restart should be False when nothing changed"
-    )
+    assert deployer.need_restart is False, "need_restart should be False when nothing changed"
 
 
-def test_install_unsupported_arch_falls_back_to_apt(
+def test_install_unsupported_arch_raises(
     deployer, patch_blocked, mock_files_put, track_shell, monkeypatch
 ):
-    # For unsupported architectures, all fact lookups return the arch string.
     monkeypatch.setattr(
         dovecot_deployer,
         "host",
-        SimpleNamespace(get_fact=lambda cls: "riscv64"),
+        make_host(
+            (dovecot_deployer.Arch, "riscv64"),
+            ((Command, dovecot_deployer.VERSION_ID_CMD), 'VERSION_ID="12"'),
+        ),
     )
-    apt_calls = []
 
-    # Mirrors apt.packages() return value: OperationMeta with .changed property.
-    # Only lmtpd triggers a change to verify |= accumulation of changed flags.
-    def fake_apt(**kwargs):
-        apt_calls.append(kwargs)
-        changed = "lmtpd" in kwargs["packages"][0]
-        return SimpleNamespace(changed=changed)
+    # we never fall back to the pinned distro package
+    with pytest.raises(ValueError, match="no dovecot build for dovecot-core"):
+        deployer.install()
 
-    monkeypatch.setattr(dovecot_deployer.apt, "packages", fake_apt)
-
-    deployer.install()
-
-    actual_pkgs = [c["packages"] for c in apt_calls]
-    assert actual_pkgs == [["dovecot-core"], ["dovecot-imapd"], ["dovecot-lmtpd"]], (
-        f"expected apt install of core/imapd/lmtpd, got {actual_pkgs}"
-    )
-    assert track_shell == [], "should not run dpkg for unsupported arch"
-    assert deployer.need_restart is True, (
-        "need_restart should be True when apt installed a package"
-    )
+    assert track_shell == [], "should not run apt-get for unsupported arch"
 
 
 def test_install_runs_dpkg_when_packages_need_download(
@@ -198,6 +207,7 @@ def test_install_runs_dpkg_when_packages_need_download(
         make_host(
             (dovecot_deployer.DebPackages, {}),
             (dovecot_deployer.Arch, "x86_64"),
+            ((Command, dovecot_deployer.VERSION_ID_CMD), 'VERSION_ID="12"'),
         ),
     )
     monkeypatch.setattr(
@@ -213,17 +223,15 @@ def test_install_runs_dpkg_when_packages_need_download(
 
     deployer.install()
 
-    assert len(track_shell) == 1, (
-        f"expected one server.shell() call for dpkg install, got {len(track_shell)}"
-    )
+    assert len(track_shell) == 1, f"expected one server.shell() call for dpkg install, got {len(track_shell)}"
     cmds = track_shell[0]["commands"]
-    assert len(cmds) == 3, f"expected 3 dpkg/apt commands, got: {cmds}"
-    assert cmds[0].startswith("dpkg --force-confdef --force-confold -i ")
-    assert "apt-get -y --fix-broken install" in cmds[1]
-    assert cmds[2].startswith("dpkg --force-confdef --force-confold -i ")
-    assert deployer.need_restart is True, (
-        "need_restart should be True after dpkg install"
-    )
+    assert len(cmds) == 1, f"expected single apt-get install command, got: {cmds}"
+    assert "apt-get install -y" in cmds[0]
+    assert '-o Dpkg::Options::="--force-confdef"' in cmds[0]
+    assert '-o Dpkg::Options::="--force-confold"' in cmds[0]
+    assert "--allow-downgrades" in cmds[0]
+    assert ".deb" in cmds[0]
+    assert deployer.need_restart is True, "need_restart should be True after dpkg install"
 
 
 def test_pick_url_falls_back_on_primary_error(monkeypatch):
@@ -232,6 +240,43 @@ def test_pick_url_falls_back_on_primary_error(monkeypatch):
 
     monkeypatch.setattr(dovecot_deployer.urllib.request, "urlopen", raise_error)
     result = dovecot_deployer._pick_url("http://primary", "http://fallback")
-    assert result == "http://fallback", (
-        f"should fall back when primary fails, got {result!r}"
+    assert result == "http://fallback", f"should fall back when primary fails, got {result!r}"
+
+
+def test_install_fails_on_unsupported_debian_version(deployer, patch_blocked, monkeypatch):
+    monkeypatch.setattr(
+        dovecot_deployer,
+        "host",
+        make_host(
+            (dovecot_deployer.Arch, "x86_64"),
+            ((Command, dovecot_deployer.VERSION_ID_CMD), 'VERSION_ID="99"'),
+        ),
     )
+
+    with pytest.raises(ValueError, match="no dovecot build for dovecot-core on deb99"):
+        deployer.install()
+
+
+@pytest.mark.parametrize(
+    "version_line", ["", None, "ID=debian"], ids=["empty", "none", "no-version-id"]
+)
+def test_parse_version_id_raises_without_version_id(version_line):
+    with pytest.raises(ValueError, match="cannot determine Debian release"):
+        dovecot_deployer._parse_version_id(version_line)
+
+
+@pytest.mark.parametrize("deb_release", [12, 13])
+def test_parse_version_id(deb_release):
+    parsed = dovecot_deployer._parse_version_id(f'VERSION_ID="{deb_release}"\n')
+    assert parsed == deb_release
+
+
+def test_dovecot_sha256_covers_all_packages_per_release():
+    """Every release in the table needs all three packages on both arches."""
+    table = dovecot_deployer.DOVECOT_SHA256
+    expected = {
+        (arch, pkg) for arch in ("amd64", "arm64") for pkg in ("core", "imapd", "lmtpd")
+    }
+    for release in {r for _, r, _ in table}:
+        got = {(arch, pkg) for arch, r, pkg in table if r == release}
+        assert got == expected, f"deb{release} incomplete: {sorted(expected - got)}"

doc/source/getting_started.rst

@@ -199,6 +199,93 @@ creating addresses, login with ssh to the deployment machine and run:
 Chatmail address creation will be denied while this file is present.
 
 
+.. _system-limits:
+
+Configurable System Limits
+--------------------------
+
+Limits for auto-rejecting address creation
+..........................................
+
+A relay refuses creation of new addresses
+when the machine runs low on resources,
+but existing addresses keep working.
+
+Three ``chatmail.ini`` settings control this,
+shown here with their defaults::
+
+    max_load_1m = 5
+    min_available_memory = 200M
+    min_free_disk_space = 1G
+
+- ``max_load_1m`` is the maximum 1-minute load average,
+  as reported by ``uptime``;
+  it counts processes waiting for disk I/O as well as for CPU.
+  It is deliberately not scaled by the number of CPUs
+  because I/O rather than CPU is what typically limits a relay.
+
+- ``min_available_memory`` is the minimum memory available without swapping.
+
+- ``min_free_disk_space`` is the minimum free disk space
+  on the file system holding the mailboxes.
+
+The defaults suit the small machine described in
+`Minimal requirements and prerequisites`_.
+
+.. note::
+
+   If you run a bigger machine,
+   raise ``max_load_1m`` after watching ``uptime`` under typical load.
+
+Rejections are logged by the ``doveauth`` service,
+so you can check whether a limit is set too tightly::
+
+    journalctl -u doveauth --grep 'registration rejected'
+
+If ``mtail_address`` is set, rejections are also counted
+in the ``rejected_registrations`` metric.
+
+
+Overall IMAP and SMTP connection limits
+.......................................
+
+Two further settings bound how many connections
+the relay accepts at all, again shown with their defaults::
+
+    max_imap_connections = 10000
+    max_smtp_connections = 1000
+
+``max_imap_connections`` becomes the Dovecot imap process limit,
+and ``max_smtp_connections`` the Postfix process limit
+on each of the submission and smtps ports.
+A single client IP may use up to a fifth of ``max_smtp_connections``.
+Each connection costs memory,
+so these limits defend the relay against running out of RAM.
+
+Unless ``imap_compress`` is enabled,
+an IMAP connection that is idle for ``imap_hibernate_timeout``
+is handed over to the ``imap-hibernate`` process
+and does not count towards ``max_imap_connections``,
+which is why a relay can serve far more IMAP clients
+than this setting suggests.
+
+If you run a large relay with 10k or 100k's of addresses,
+check current connection counts before upgrading
+and set the limits accordingly.
+
+To see how close a running relay is to these two limits,
+copy ``scripts/check-connections.sh`` from the relay repository
+onto the relay and run it there::
+
+    imap             5  ports 143,993   (max_imap_connections)
+                     5  dovecot sessions, 0 of them in an active imap process
+    submission       0  ports 465,587   (max_smtp_connections per port)
+    incoming         0  port 25         (from other relays, no chatmail.ini limit)
+
+It counts established sockets with ``ss``
+and cross-checks the IMAP number against ``doveadm who``.
+
+
 Running a relay with self-signed certificates
 ----------------------------------------------
 

scripts/check-connections.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+#
+# Show current IMAP and SMTP connections on a chatmail relay,
+# to compare against the max_imap_connections and max_smtp_connections
+# settings in chatmail.ini.  Run this on the relay itself.
+set -e
+
+# Count established TCP connections whose local port is one of the arguments.
+established() {
+    filter=$(printf 'sport = :%s or ' "$@")
+    ss -Htn state established "( ${filter% or } )" | wc -l
+}
+
+# doveadm prints a header line and then one line per logged-in user,
+# with that user's number of connections in the second column.
+sessions=$(doveadm who | awk 'NR > 1 { n += $2 } END { print n + 0 }')
+
+# Unless imap_compress is enabled, connections idle for
+# imap_hibernate_timeout are handed over to the imap-hibernate
+# process, so they cost no imap process while idle.
+active=$(pgrep -x imap | wc -l)
+
+printf 'imap        %6d  ports 143,993   (max_imap_connections)\n' "$(established 143 993)"
+printf '            %6d  dovecot sessions, %d of them in an active imap process\n' "$sessions" "$active"
+printf 'submission  %6d  ports 465,587   (max_smtp_connections per port)\n' "$(established 465 587)"
+printf 'incoming    %6d  port 25         (from other relays, no chatmail.ini limit)\n' "$(established 25)"