feat: serve an APPVERSIONS.json index file to clients via IMAP metadata

This is designed to help implement self-updating APKs (and later other clients),
see counterpart https://github.com/chatmail/core/pull/8557

.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:"
 

APPVERSIONS.json

@@ -0,0 +1,15 @@
+{
+  "clients": [
+    {
+      "clientId": "deltachat",
+      "sources": [
+        {
+          "sourceId": "gplay",
+          "versionInteger": 754,
+          "versionString": "2.57.0",
+          "downloadUrl": "https://github.com/deltachat/deltachat-android/releases/download/v2.57.0/deltachat-gplay-release-2.57.0.apk"
+        }
+      ]
+    }
+  ]
+}

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

CONTRIBUTING.md

@@ -5,3 +5,6 @@ We use [git-cliff] to generate the changelog from commit messages before the rel
 
 [Conventional Commits]: https://www.conventionalcommits.org/
 [git-cliff]: https://git-cliff.org/
+
+To update client app version information,
+edit [APPVERSIONS.json](APPVERSIONS.json).

README.md

@@ -1,7 +1,7 @@
 
 # Chatmail relays for end-to-end encrypted email
 
-Chatmail relay servers are interoperable Mail Transport Agents (MTAs) designed for: 
+Chatmail relay servers are interoperable Mail Transport Agents (MTAs) designed for:
 
 -  **Zero State:** no private data or metadata collected, messages are auto-deleted, low disk usage
 
@@ -18,7 +18,7 @@ Chatmail relay servers are interoperable Mail Transport Agents (MTAs) designed f
 -  **Reliable Federation and Decentralization:** No spam or IP reputation checks, federating
    depends on established IETF standards and protocols.
 
-This repository contains everything needed to setup a ready-to-use chatmail relay on an ssh-reachable host. 
+This repository contains everything needed to setup a ready-to-use chatmail relay on an ssh-reachable host.
 For getting started and more information please refer to the web version of this repositories' documentation at
 
 [https://chatmail.at/doc/relay](https://chatmail.at/doc/relay)

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

@@ -63,6 +63,9 @@ class Config:
         self.turn_socket_path = params.pop(
             "turn_socket_path", "/run/chatmail-turn/turn.socket"
         )
+        self.appversions_path = Path(
+            params.pop("appversions_path", "/usr/local/lib/chatmaild/appversions.json")
+        )
         iroh_relay = params.pop("iroh_relay", None)
         if iroh_relay is None:
             self.iroh_relay = "https://" + raw_domain
@@ -75,6 +78,16 @@ 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/metadata.py

@@ -1,3 +1,4 @@
+import json
 import logging
 import socket
 import sys
@@ -18,6 +19,18 @@ def turn_credentials(turn_socket_path):
             return file.readline().decode("utf-8").strip()
 
 
+def read_appversions(path):
+    try:
+        data = json.loads(path.read_bytes())
+    except FileNotFoundError:
+        return None
+    except (OSError, ValueError):
+        logging.exception(f"failed to read {path}")
+        return None
+    # the dict protocol is line-based, keep the value single-line
+    return json.dumps(data, separators=(",", ":"))
+
+
 def _is_valid_token_timestamp(timestamp, now):
     # Token if invalid after 90 days
     # or if the timestamp is in the future.
@@ -94,6 +107,7 @@ class MetadataDictProxy(DictProxy):
         iroh_relay=None,
         turn_hostname=None,
         turn_socket_path=None,
+        appversions_path=None,
     ):
         super().__init__()
         self.notifier = notifier
@@ -101,6 +115,7 @@ class MetadataDictProxy(DictProxy):
         self.iroh_relay = iroh_relay
         self.turn_hostname = turn_hostname
         self.turn_socket_path = turn_socket_path
+        self.appversions_path = appversions_path
 
     def handle_lookup(self, parts):
         # Lpriv/43f5f508a7ea0366dff30200c15250e3/devicetoken\tlkj123poi@c2.testrun.org
@@ -125,6 +140,9 @@ class MetadataDictProxy(DictProxy):
                         case "maxsmtprecipients":
                             # postfix default  (see "postconf smtpd_recipient_limit")
                             return "O1000\n"
+                        case "appversions" if self.appversions_path:
+                            value = read_appversions(self.appversions_path)
+                            return f"O{value}\n" if value else "N\n"
 
         logging.warning(f"lookup ignored: {parts!r}")
         return "N\n"
@@ -170,6 +188,7 @@ def main():
         iroh_relay=iroh_relay,
         turn_hostname=mail_domain,
         turn_socket_path=socket_path,
+        appversions_path=config.appversions_path,
     )
 
     dictproxy.serve_forever_from_socket(socket)

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 +45,11 @@ 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 str(example_config.appversions_path) == (
+        "/usr/local/lib/chatmaild/appversions.json"
+    )
     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_metadata.py

@@ -7,6 +7,7 @@ import requests
 from chatmaild.metadata import (
     Metadata,
     MetadataDictProxy,
+    read_appversions,
 )
 from chatmaild.notifier import (
     Notifier,
@@ -369,6 +370,17 @@ def test_iroh_relay(dictproxy):
     assert wfile.getvalue() == b"Ohttps://example.org/\n"
 
 
+def test_read_appversions(tmp_path):
+    path = tmp_path.joinpath("appversions.json")
+    assert read_appversions(path) is None
+
+    path.write_text('{\n  "clients": []\n}')
+    assert read_appversions(path) == '{"clients":[]}'
+
+    path.write_text("bad json")
+    assert read_appversions(path) is None
+
+
 def test_legacy_token_migration(metadata, testaddr):
     with metadata.get_metadata_dict(testaddr).modify() as data:
         data[metadata.DEVICETOKEN_KEY] = ["oldtoken1", "oldtoken2"]

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/deployers.py

@@ -35,7 +35,7 @@ from .nginx.deployer import NginxDeployer
 from .opendkim.deployer import OpendkimDeployer
 from .postfix.deployer import PostfixDeployer
 from .selfsigned.deployer import SelfSignedTlsDeployer
-from .www import build_webpages, find_merge_conflict, get_paths
+from .www import build_webpages, find_merge_conflict, get_paths, get_reporoot
 
 
 class Port(FactBase):
@@ -126,6 +126,11 @@ def _configure_remote_venv_with_chatmaild(deployer, config) -> None:
         dest=remote_chatmail_inipath,
     )
 
+    deployer.put_file(
+        src=get_reporoot().joinpath("APPVERSIONS.json").open("rb"),
+        dest=str(config.appversions_path),
+    )
+
     deployer.remove_file("/etc/cron.d/chatmail-metrics")
     deployer.remove_file("/var/www/html/metrics")
 
@@ -138,9 +143,22 @@ 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 +184,6 @@ 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 +258,13 @@ 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 +375,8 @@ 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",
@@ -504,10 +525,10 @@ 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

@@ -34,7 +34,7 @@ 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)

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/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++

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/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

cmdeploy/src/cmdeploy/tests/online/test_2_deltachat.py

@@ -51,6 +51,16 @@ class TestMetadataTokens:
         assert res == b"1111 2222"
         assert b"Getmetadata completed" in client.readline()
 
+    def test_get_appversions(self, imap_mailbox):
+        "get app version information shipped with the relay"
+        client = imap_mailbox.client
+        client.send(b'a01 GETMETADATA "" /shared/vendor/deltachat/appversions\n')
+        res = client.readline()
+        assert res[:1] == b"*"
+        res = client.readline().strip().rstrip(b")")
+        assert b'"clients":' in res
+        assert b"Getmetadata completed" in client.readline()
+
 
 class TestEndToEndDeltaChat:
     "Tests that use Delta Chat accounts on the chat mail instance."

cmdeploy/src/cmdeploy/tests/test_appversions.py

@@ -0,0 +1,20 @@
+import json
+
+from cmdeploy.www import get_reporoot
+
+ALLOWED_URL_PREFIXES = (
+    "https://github.com/deltachat/",
+    "https://download.delta.chat/",
+)
+
+
+def test_appversions_schema():
+    data = json.loads(get_reporoot().joinpath("APPVERSIONS.json").read_text())
+    assert data["clients"]
+    for client in data["clients"]:
+        assert isinstance(client["clientId"], str)
+        for source in client["sources"]:
+            assert isinstance(source["sourceId"], str)
+            assert isinstance(source["versionInteger"], int)
+            assert isinstance(source["versionString"], str)
+            assert source["downloadUrl"].startswith(ALLOWED_URL_PREFIXES)

cmdeploy/src/cmdeploy/www.py

@@ -35,8 +35,12 @@ def prepare_template(source):
     return render_vars, page_layout
 
 
+def get_reporoot() -> Path:
+    return (Path(__file__).resolve() / "../../../../").resolve()
+
+
 def get_paths(config) -> (Path, Path, Path):
-    reporoot = (Path(__file__).resolve() / "../../../../").resolve()
+    reporoot = get_reporoot()
     www_path = Path(config.www_folder)
     # if www_folder was not set, use default directory
     if config.www_folder == "":

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
 ----------------------------------------------
 

doc/source/overview.rst

@@ -249,6 +249,28 @@ Fresh chatmail addresses have a mailbox directory that contains:
    directories will typically be empty unless the user of that address
    hasn’t been online for a while.
 
+App version information (experimental)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A chatmail relay ships the repository's
+`APPVERSIONS.json <https://github.com/chatmail/relay/blob/main/APPVERSIONS.json>`_
+and serves its content under the IMAP METADATA key
+``/shared/vendor/deltachat/appversions``.
+Chat apps installed outside of app stores read this key
+to learn about updates and where to download them.
+The mechanism is experimental and may change.
+
+The file travels with the normal deploy:
+update the repository checkout and run ``cmdeploy run``.
+Local modifications of ``APPVERSIONS.json`` are deployed as-is,
+so you can serve your own app version information,
+including links to app downloads.
+
+Note that as of August 2026, only Delta Chat Android is beginning
+to support discovering app versions from relays.
+Consumers of relay-provided app version information
+need to verify themselves that downloaded app files are valid.
+
 Active ports
 ~~~~~~~~~~~~
 

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)"