retain "config.mail_domain" as the domain part of @ email addresses, so for ipv4 relays "[1.2.3.4]" and introduce config.ipv4_relay and config.mail_domain_bare helpers.

.github/workflows/ci-no-dns.yaml

@@ -0,0 +1,35 @@
+name: No-DNS
+
+on:
+  # Triggers when a PR is merged into main or a direct push occurs
+  push:
+    branches: [ "main" ]
+    
+  # Triggers for any PR (and its subsequent commits) targeting the main branch
+  pull_request:
+    branches: [ "main" ]
+
+# Newest push wins: Prevents multiple runs from clashing and wasting runner efforts
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+
+jobs:
+  no-dns:
+    name: LXC deploy and test
+    uses: chatmail/cmlxc/.github/workflows/lxc-test.yml@d39fe34c39cee6d760c3479325e8dc82b66a8928
+    with:
+      cmlxc_commands: |
+        cmlxc init 
+        # single cmdeploy relay test
+        cmlxc -v deploy-cmdeploy --source ./repo --ipv4-only --no-dns cm0
+        cmlxc -v test-cmdeploy --no-dns cm0
+
+        # cross cmdeploy relay test
+        cmlxc -v deploy-cmdeploy --source ./repo cm1
+        cmlxc -v test-cmdeploy --no-dns cm0 cm1
+
+        # cross cmdeploy/madmail relay tests
+        cmlxc -v deploy-madmail mad0
+        cmlxc -v test-cmdeploy --no-dns cm0 mad0

.github/workflows/ci.yaml

@@ -1,4 +1,4 @@
-name: Run unit-tests and container-based deploy+test verification
+name: CI
 
 on:
   # Triggers when a PR is merged into main or a direct push occurs
@@ -57,7 +57,7 @@ jobs:
 
   lxc-test:
     name: LXC deploy and test
-    uses: chatmail/cmlxc/.github/workflows/lxc-test.yml@v0.10.0
+    uses: chatmail/cmlxc/.github/workflows/lxc-test.yml@d39fe34c39cee6d760c3479325e8dc82b66a8928
     with:
       cmlxc_commands: |
         cmlxc init 

chatmaild/pyproject.toml

@@ -10,6 +10,7 @@ dependencies = [
   "filelock",
   "requests",
   "crypt-r >= 3.13.1 ; python_version >= '3.11'",
+  "domain-validator",
 ]
 
 [tool.setuptools]

chatmaild/src/chatmaild/config.py

@@ -1,6 +1,8 @@
+import ipaddress
 from pathlib import Path
 
 import iniconfig
+from domain_validator import DomainValidator
 
 from chatmaild.user import User
 
@@ -19,7 +21,19 @@ def read_config(inipath):
 class Config:
     def __init__(self, inipath, params):
         self._inipath = inipath
-        self.mail_domain = params["mail_domain"]
+        raw_domain = params["mail_domain"]
+        self.mail_domain_bare = raw_domain
+
+        if is_valid_ipv4(raw_domain):
+            self.ipv4_relay = raw_domain
+            self.mail_domain = f"[{raw_domain}]"
+            self.postfix_myhostname = ipaddress.IPv4Address(raw_domain).reverse_pointer
+        else:
+            DomainValidator().validate_domain_re(raw_domain)
+            self.ipv4_relay = None
+            self.mail_domain = raw_domain
+            self.postfix_myhostname = raw_domain
+
         self.max_user_send_per_minute = int(params.get("max_user_send_per_minute", 60))
         self.max_user_send_burst_size = int(params.get("max_user_send_burst_size", 10))
         self.max_mailbox_size = params["max_mailbox_size"]
@@ -53,7 +67,7 @@ class Config:
         self.imap_rawlog = params.get("imap_rawlog", "false").lower() == "true"
         self.imap_compress = params.get("imap_compress", "false").lower() == "true"
         if "iroh_relay" not in params:
-            self.iroh_relay = "https://" + params["mail_domain"]
+            self.iroh_relay = "https://" + raw_domain
             self.enable_iroh_relay = True
         else:
             self.iroh_relay = params["iroh_relay"].strip()
@@ -79,17 +93,17 @@ class Config:
                 )
             self.tls_cert_mode = "external"
             self.tls_cert_path, self.tls_key_path = parts
-        elif self.mail_domain.startswith("_"):
+        elif raw_domain.startswith("_") or self.ipv4_relay:
             self.tls_cert_mode = "self"
             self.tls_cert_path = "/etc/ssl/certs/mailserver.pem"
             self.tls_key_path = "/etc/ssl/private/mailserver.key"
         else:
             self.tls_cert_mode = "acme"
-            self.tls_cert_path = f"/var/lib/acme/live/{self.mail_domain}/fullchain"
-            self.tls_key_path = f"/var/lib/acme/live/{self.mail_domain}/privkey"
+            self.tls_cert_path = f"/var/lib/acme/live/{raw_domain}/fullchain"
+            self.tls_key_path = f"/var/lib/acme/live/{raw_domain}/privkey"
 
         # deprecated option
-        mbdir = params.get("mailboxes_dir", f"/home/vmail/mail/{self.mail_domain}")
+        mbdir = params.get("mailboxes_dir", f"/home/vmail/mail/{raw_domain}")
         self.mailboxes_dir = Path(mbdir.strip())
 
         # old unused option (except for first migration from sqlite to maildir store)
@@ -175,3 +189,27 @@ def get_default_config_content(mail_domain, **overrides):
                 lines.append(line)
         content = "\n".join(lines)
     return content
+
+
+def is_valid_ipv4(address: str) -> bool:
+    """Check if a mail_domain is an IPv4 address."""
+    try:
+        ipaddress.IPv4Address(address)
+        return True
+    except ValueError:
+        return False
+
+
+
+def format_arpa_address(address: str) -> str:
+    if is_valid_ipv4(address):
+        return ipaddress.IPv4Address(address).reverse_pointer
+    DomainValidator().validate_domain_re(address)
+    return address
+
+
+def format_mail_domain(raw_domain: str) -> str:
+    if is_valid_ipv4(raw_domain):
+        return f"[{raw_domain}]"
+    DomainValidator().validate_domain_re(raw_domain)
+    return raw_domain

chatmaild/src/chatmaild/newemail.py

@@ -2,7 +2,6 @@
 
 """CGI script for creating new accounts."""
 
-import ipaddress
 import json
 import secrets
 import string
@@ -15,16 +14,6 @@ ALPHANUMERIC = string.ascii_lowercase + string.digits
 ALPHANUMERIC_PUNCT = string.ascii_letters + string.digits + string.punctuation
 
 
-def wrap_ip(host):
-    if host.startswith("[") and host.endswith("]"):
-        return host
-    try:
-        ipaddress.ip_address(host)
-        return f"[{host}]"
-    except ValueError:
-        return host
-
-
 def create_newemail_dict(config: Config):
     user = "".join(
         secrets.choice(ALPHANUMERIC) for _ in range(config.username_max_length)
@@ -33,16 +22,22 @@ def create_newemail_dict(config: Config):
         secrets.choice(ALPHANUMERIC_PUNCT)
         for _ in range(config.password_min_length + 3)
     )
-    return dict(email=f"{user}@{wrap_ip(config.mail_domain)}", password=f"{password}")
+    return dict(email=f"{user}@{config.mail_domain}", password=f"{password}")
 
 
-def create_dclogin_url(email, password):
+def create_dclogin_url(config, email, password):
     """Build a dclogin: URL with credentials and self-signed cert acceptance.
 
     Uses ic=3 (AcceptInvalidCertificates) so chatmail clients
     can connect to servers with self-signed TLS certificates.
     """
-    return f"dclogin:{quote(email, safe='@')}?p={quote(password, safe='')}&v=1&ic=3"
+    if config.ipv4_relay:
+        imap_host = "&ih=" + config.ipv4_relay
+        smtp_host = "&sh=" + config.ipv4_relay
+    else:
+        imap_host = ""
+        smtp_host = ""
+    return f"dclogin:{quote(email, safe='@[]')}?p={quote(password, safe='')}&v=1{imap_host}{smtp_host}&ic=3"
 
 
 def print_new_account():
@@ -51,7 +46,9 @@ def print_new_account():
 
     result = dict(email=creds["email"], password=creds["password"])
     if config.tls_cert_mode == "self":
-        result["dclogin_url"] = create_dclogin_url(creds["email"], creds["password"])
+        result["dclogin_url"] = create_dclogin_url(
+            config, creds["email"], creds["password"]
+        )
 
     print("Content-Type: application/json")
     print("")

chatmaild/src/chatmaild/tests/plugin.py

@@ -31,6 +31,11 @@ def example_config(make_config):
     return make_config("chat.example.org")
 
 
+@pytest.fixture
+def ipv4_config(make_config):
+    return make_config("1.3.3.7")
+
+
 @pytest.fixture
 def maildomain(example_config):
     return example_config.mail_domain

chatmaild/src/chatmaild/tests/test_config.py

@@ -1,6 +1,14 @@
+from contextlib import nullcontext as does_not_raise
+
 import pytest
 
-from chatmaild.config import parse_size_mb, read_config
+from chatmaild.config import (
+    format_arpa_address,
+    format_mail_domain,
+    is_valid_ipv4,
+    parse_size_mb,
+    read_config,
+)
 
 
 def test_read_config_basic(example_config):
@@ -13,6 +21,12 @@ def test_read_config_basic(example_config):
     example_config = read_config(inipath)
     assert example_config.max_user_send_per_minute == 37
     assert example_config.mail_domain == "chat.example.org"
+    assert example_config.ipv4_relay is None
+
+
+def test_read_config_ipv4(ipv4_config):
+    assert ipv4_config.ipv4_relay == "1.3.3.7"
+    assert ipv4_config.mail_domain == "[1.3.3.7]"
 
 
 def test_read_config_basic_using_defaults(tmp_path, maildomain):
@@ -135,3 +149,45 @@ def test_max_mailbox_size_mb(make_config):
     config = make_config("chat.example.org")
     assert config.max_mailbox_size == "500M"
     assert config.max_mailbox_size_mb == 500
+
+
+@pytest.mark.parametrize(
+    ["input", "result"],
+    [
+        ("example.org", False),
+        ("1.3.3.7", True),
+        ("fe::1", False),
+        ("ad.1e.dag.adf", False),
+        ("12394142", False),
+    ],
+)
+def test_is_valid_ipv4(input, result):
+    assert result == is_valid_ipv4(input)
+
+
+@pytest.mark.parametrize(
+    ["input", "result", "exception"],
+    [
+        ("example.org", "example.org", does_not_raise()),
+        ("1.3.3.7", "7.3.3.1.in-addr.arpa", does_not_raise()),
+        ("fe::1", None, pytest.raises(ValueError)),
+        ("12394142", None, pytest.raises(ValueError)),
+    ],
+)
+def test_format_arpa_address(input, result, exception):
+    with exception:
+        assert result == format_arpa_address(input)
+
+
+@pytest.mark.parametrize(
+    ["input", "result", "exception"],
+    [
+        ("example.org", "example.org", does_not_raise()),
+        ("1.3.3.7", "[1.3.3.7]", does_not_raise()),
+        ("fe::1", None, pytest.raises(ValueError)),
+        ("12394142", None, pytest.raises(ValueError)),
+    ],
+)
+def test_format_mail_domain(input, result, exception):
+    with exception:
+        assert result == format_mail_domain(input)

chatmaild/src/chatmaild/tests/test_newmail.py

@@ -19,24 +19,35 @@ def test_create_newemail_dict(example_config):
     assert ac1["password"] != ac2["password"]
 
 
-def test_create_newemail_dict_ip(make_config):
-    config = make_config("1.2.3.4")
-    ac = create_newemail_dict(config)
-    assert ac["email"].endswith("@[1.2.3.4]")
+def test_create_newemail_dict_ip(ipv4_config):
+    ac = create_newemail_dict(ipv4_config)
+    assert ac["email"].endswith("@[1.3.3.7]")
 
 
-def test_create_dclogin_url():
-    url = create_dclogin_url("user@example.org", "p@ss w+rd")
+def test_create_dclogin_url(example_config):
+    addr = "user@example.org"
+    password = "p@ss w+rd"
+    url = create_dclogin_url(example_config, addr, password)
     assert url.startswith("dclogin:")
     assert "v=1" in url
     assert "ic=3" in url
 
-    assert "user@example.org" in url
+    assert addr in url
     # password special chars must be encoded
     assert "p%40ss" in url
     assert "w%2Brd" in url
 
 
+def test_create_dclogin_url_ipv4(ipv4_config):
+    addr = "user@[1.3.3.7]"
+    password = "p@ss w+rd"
+    url = create_dclogin_url(ipv4_config, addr, password)
+    assert url.startswith("dclogin:")
+    assert "v=1" in url
+    assert "ic=3" in url
+    assert addr in url
+
+
 def test_print_new_account(capsys, monkeypatch, maildomain, tmpdir, example_config):
     monkeypatch.setattr(chatmaild.newemail, "CONFIG_PATH", str(example_config._inipath))
     print_new_account()

cmdeploy/src/cmdeploy/cmdeploy.py

@@ -87,10 +87,12 @@ def run_cmd_options(parser):
 def run_cmd(args, out):
     """Deploy chatmail services on the remote server."""
 
-    ssh_host = args.ssh_host if args.ssh_host else args.config.mail_domain
+    ssh_host = args.ssh_host if args.ssh_host else args.config.mail_domain_bare
     sshexec = get_sshexec(ssh_host)
     require_iroh = args.config.enable_iroh_relay
     strict_tls = args.config.tls_cert_mode == "acme"
+    if args.config.ipv4_relay:
+        args.dns_check_disabled = True
     if not args.dns_check_disabled:
         remote_data = dns.get_initial_remote_data(sshexec, args.config.mail_domain)
         if not dns.check_initial_remote_data(remote_data, strict_tls=strict_tls, print=out.red):
@@ -119,6 +121,8 @@ def run_cmd(args, out):
         elif not args.dns_check_disabled and strict_tls and not remote_data["acme_account_url"]:
             out.red("Deploy completed but letsencrypt not configured")
             out.red("Run 'cmdeploy run' again")
+        elif args.config.ipv4_relay:
+            out.green("Deploy completed.")
         else:
             out.green("Deploy completed, call `cmdeploy dns` next.")
         return 0
@@ -140,6 +144,10 @@ def dns_cmd_options(parser):
 
 def dns_cmd(args, out):
     """Check DNS entries and optionally generate dns zone file."""
+    if args.config.ipv4_relay:
+        ipv4 = args.config.ipv4_relay
+        print(f"[WARNING] {ipv4} is not a domain, skipping DNS checks.")
+        return 0
     ssh_host = args.ssh_host if args.ssh_host else args.config.mail_domain
     sshexec = get_sshexec(ssh_host, verbose=args.verbose)
     tls_cert_mode = args.config.tls_cert_mode
@@ -177,7 +185,7 @@ def status_cmd_options(parser):
 def status_cmd(args, out):
     """Display status for online chatmail instance."""
 
-    ssh_host = args.ssh_host if args.ssh_host else args.config.mail_domain
+    ssh_host = args.ssh_host if args.ssh_host else args.config.mail_domain_bare
     sshexec = get_sshexec(ssh_host, verbose=args.verbose)
 
     out.green(f"chatmail domain: {args.config.mail_domain}")

cmdeploy/src/cmdeploy/deployers.py

@@ -468,7 +468,7 @@ class ChatmailVenvDeployer(Deployer):
 
     def configure(self):
         _configure_remote_venv_with_chatmaild(self.config)
-        configure_remote_units(self.config.mail_domain, self.units)
+        configure_remote_units(self.config.mail_domain_bare, self.units)
 
     def activate(self):
         activate_remote_units(self.units)
@@ -584,7 +584,7 @@ def deploy_chatmail(config_path: Path, disable_mail: bool, website_only: bool) -
     """
     config = read_config(config_path)
     check_config(config)
-    mail_domain = config.mail_domain
+    bare_host = config.mail_domain_bare
 
     if website_only:
         Deployment().perform_stages([WebsiteDeployer(config)])
@@ -635,7 +635,7 @@ def deploy_chatmail(config_path: Path, disable_mail: bool, website_only: bool) -
                     )
                     exit(1)
 
-    tls_deployer = get_tls_deployer(config, mail_domain)
+    tls_deployer = get_tls_deployer(config, bare_host)
 
     all_deployers = [
         ChatmailDeployer(config),
@@ -643,13 +643,13 @@ def deploy_chatmail(config_path: Path, disable_mail: bool, website_only: bool) -
         FiltermailDeployer(),
         JournaldDeployer(),
         UnboundDeployer(config),
-        TurnDeployer(mail_domain),
+        TurnDeployer(bare_host),
         IrohDeployer(config.enable_iroh_relay),
         tls_deployer,
         WebsiteDeployer(config),
         ChatmailVenvDeployer(config),
         MtastsDeployer(),
-        OpendkimDeployer(mail_domain),
+        OpendkimDeployer(config.mail_domain),
         # Dovecot should be started before Postfix
         # because it creates authentication socket
         # required by Postfix.

cmdeploy/src/cmdeploy/dovecot/deployer.py

@@ -73,7 +73,7 @@ class DovecotDeployer(Deployer):
         )
 
     def configure(self):
-        configure_remote_units(self.config.mail_domain, self.units)
+        configure_remote_units(self.config.mail_domain_bare, self.units)
         config_restart, self.daemon_reload = _configure_dovecot(self.config)
         self.need_restart |= config_restart
 

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

@@ -7,6 +7,7 @@ listen = 0.0.0.0
 protocols = imap lmtp
 
 auth_mechanisms = plain
+auth_username_chars = abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890.-_@[]
 
 {% if debug == true %}
 auth_verbose = yes

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

@@ -54,14 +54,16 @@ smtpd_tls_exclude_ciphers = aNULL, RC4, MD5, DES
 tls_preempt_cipherlist = yes
 
 smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
-myhostname = {{ config.mail_domain }}
+myhostname = {{ config.postfix_myhostname }}
 alias_maps = hash:/etc/aliases
 alias_database = hash:/etc/aliases
 
-# Postfix does not deliver mail for any domain by itself.
-# Primary domain is listed in `virtual_mailbox_domains` instead
-# and handed over to Dovecot.
-mydestination =
+# When postfix receives mail for $mydestination,
+# it hands it over to dovecot via $local_transport.
+mydestination = {{ config.mail_domain }}
+local_transport = lmtp:unix:private/dovecot-lmtp
+# postfix doesn't check whether local users exist or not:
+local_recipient_maps =
 
 relayhost = 
 {% if disable_ipv6 %}
@@ -79,8 +81,6 @@ inet_protocols = ipv4
 inet_protocols = all
 {% endif %}
 
-virtual_transport = lmtp:unix:private/dovecot-lmtp
-virtual_mailbox_domains = {{ config.mail_domain }}
 lmtp_header_checks = regexp:/etc/postfix/lmtp_header_cleanup
 
 mua_client_restrictions = permit_sasl_authenticated, reject

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

@@ -80,8 +80,9 @@ filter    unix -        n       n       -       -       lmtp
 127.0.0.1:{{ config.postfix_reinject_port }} inet  n       -       n       -       100      smtpd
   -o syslog_name=postfix/reinject
   -o milter_macro_daemon_name=ORIGINATING
-  -o smtpd_milters=unix:opendkim/opendkim.sock
   -o cleanup_service_name=authclean
+{% if not config.ipv4_relay %}  -o smtpd_milters=unix:opendkim/opendkim.sock
+{% endif %}
 
 # Local SMTP server for reinjecting incoming filtered mail 
 127.0.0.1:{{ config.postfix_reinject_port_incoming }} inet  n       -       n       -       100      smtpd

cmdeploy/src/cmdeploy/tests/online/test_0_login.py

@@ -89,12 +89,11 @@ def test_concurrent_logins_same_account(
         assert login_results.get()
 
 
-def test_no_vrfy(cmfactory, chatmail_config):
+def test_no_vrfy(cmfactory, chatmail_config, maildomain):
     ac = cmfactory.get_online_account()
     addr = ac.get_config("addr")
-    domain = chatmail_config.mail_domain
 
-    s = smtplib.SMTP(domain)
+    s = smtplib.SMTP(maildomain)
     s.starttls()
 
     s.putcmd("vrfy", f"wrongaddress@{chatmail_config.mail_domain}")

cmdeploy/src/cmdeploy/tests/online/test_1_basic.py

@@ -8,6 +8,7 @@ import pytest
 
 from cmdeploy import remote
 from cmdeploy.cmdeploy import get_sshexec
+from chatmaild.config import is_valid_ipv4
 
 
 class TestSSHExecutor:
@@ -21,6 +22,8 @@ class TestSSHExecutor:
         assert out == out2
 
     def test_perform_initial(self, sshexec, maildomain):
+        if is_valid_ipv4(maildomain):
+            pytest.skip(f"{maildomain} is not a domain")
         res = sshexec(
             remote.rdns.perform_initial_checks, kwargs=dict(mail_domain=maildomain)
         )

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

@@ -15,7 +15,7 @@ def imap_mailbox(cmfactory, ssl_context):
     (ac1,) = cmfactory.get_online_accounts(1)
     user = ac1.get_config("addr")
     password = ac1.get_config("mail_pw")
-    host = user.split("@")[1]
+    host = user.split("@")[1].strip("[").strip("]")
     mailbox = imap_tools.MailBox(host, ssl_context=ssl_context)
     mailbox.login(user, password)
     mailbox.dc_ac = ac1
@@ -178,7 +178,7 @@ def test_hide_senders_ip_address(cmfactory, ssl_context):
     chat.send_text("testing submission header cleanup")
     user2.wait_for_incoming_msg()
     addr = user2.get_config("addr")
-    host = addr.split("@")[1]
+    host = addr.split("@")[1].strip("[").strip("]")
     pw = user2.get_config("mail_pw")
     mailbox = imap_tools.MailBox(host, ssl_context=ssl_context)
     mailbox.login(addr, pw)

cmdeploy/src/cmdeploy/tests/plugin.py

@@ -10,7 +10,8 @@ import time
 from pathlib import Path
 
 import pytest
-from chatmaild.config import read_config
+from chatmaild.config import read_config, format_mail_domain, is_valid_ipv4
+
 
 conftestdir = Path(__file__).parent
 
@@ -58,7 +59,12 @@ def chatmail_config(pytestconfig):
 
 @pytest.fixture(scope="session")
 def maildomain(chatmail_config):
-    return chatmail_config.mail_domain
+    return chatmail_config.mail_domain_bare
+
+
+@pytest.fixture(scope="session")
+def maildomain_deliverable(maildomain):
+    return format_mail_domain(maildomain)
 
 
 @pytest.fixture(scope="session")
@@ -278,7 +284,6 @@ def gencreds(chatmail_config):
 
     def gen(domain=None):
         domain = domain if domain else chatmail_config.mail_domain
-        addr_domain = f"[{domain}]" if _is_ip(domain) else domain
         while 1:
             num = next(count)
             alphanumeric = "abcdefghijklmnopqrstuvwxyz1234567890"
@@ -292,7 +297,7 @@ def gencreds(chatmail_config):
             password = "".join(
                 random.choices(alphanumeric, k=chatmail_config.password_min_length)
             )
-            yield f"{user}@{addr_domain}", f"{password}"
+            yield f"{user}@{domain}", f"{password}"
 
     return lambda domain=None: next(gen(domain))
 
@@ -317,7 +322,8 @@ class ChatmailACFactory:
 
     def _make_transport(self, domain):
         """Build a transport config dict for the given domain."""
-        addr, password = self.gencreds(domain)
+        domain_deliverable = format_mail_domain(domain)
+        addr, password = self.gencreds(domain_deliverable)
         transport = {
             "addr": addr,
             "password": password,
@@ -326,7 +332,7 @@ class ChatmailACFactory:
             "imapServer": domain,
             "smtpServer": domain,
         }
-        if self.chatmail_config.tls_cert_mode == "self":
+        if domain.startswith("_") or is_valid_ipv4(domain):
             transport["certificateChecks"] = "acceptInvalidCertificates"
         return transport
 
@@ -341,7 +347,8 @@ class ChatmailACFactory:
         accounts = []
         for _ in range(num):
             account = self.dc.add_account()
-            addr, password = self.gencreds(domain)
+            domain_deliverable = format_mail_domain(domain)
+            addr, password = self.gencreds(domain_deliverable)
             if _is_ip(domain):
                 # Use DCLOGIN scheme with explicit server hosts,
                 # matching how madmail presents its addresses to users.

cmdeploy/src/cmdeploy/tests/test_cmdeploy.py

@@ -39,6 +39,14 @@ class TestCmdline:
         out, err = capsys.readouterr()
         assert "deleting config file" in out.lower()
 
+    def test_dns_skip_on_ip(self, capsys, tmp_path, monkeypatch):
+        monkeypatch.delenv("CHATMAIL_INI", raising=False)
+        inipath = tmp_path / "chatmail.ini"
+        assert main(["init", "--config", str(inipath), "1.3.3.7"]) == 0
+        assert main(["dns", "--config", str(inipath)]) == 0
+        out, err = capsys.readouterr()
+        assert out == "[WARNING] 1.3.3.7 is not a domain, skipping DNS checks.\n"
+
 
 def test_www_folder(example_config, tmp_path):
     reporoot = importlib.resources.files(__package__).joinpath("../../../../").resolve()

doc/source/getting_started.rst

@@ -14,8 +14,6 @@ Minimal requirements and prerequisites
 
 You will need the following:
 
--  Control over a domain through a DNS provider of your choice.
-
 -  A Debian 12 **deployment server** with reachable SMTP/SUBMISSIONS/IMAPS/HTTPS ports.
    IPv6 is encouraged if available. Chatmail relay servers only require
    1GB RAM, one CPU, and perhaps 10GB storage for a few thousand active
@@ -28,6 +26,11 @@ You will need the following:
    (An ed25519 private key is required due to an `upstream bug in
    paramiko <https://github.com/paramiko/paramiko/issues/2191>`_)
 
+-  Control over a domain through a DNS provider of your choice
+   (there is experimental support for :ref:`DNS-less relays <iponly>`).
+
+
+.. _setup:
 
 Setup with ``scripts/cmdeploy``
 -------------------------------------

doc/source/index.rst

@@ -19,3 +19,4 @@ Contributions and feedback welcome through the https://github.com/chatmail/relay
     reverse_dns
     related
     faq
+    iponly

doc/source/iponly.rst

@@ -0,0 +1,29 @@
+.. _iponly:
+
+Hosting without DNS records
+===========================
+
+.. note::
+
+   This option is experimental and might change without notice.
+
+In case you don't have a domain,
+for example in a local network,
+you can run a chatmail relay with only an IPv4 address as well.
+
+To deploy a relay without a domain,
+run ``cmdeploy init`` with only the IPv4 address
+during the :ref:`installation steps <setup>`,
+for example ``cmdeploy init 13.12.23.42``.
+
+Drawbacks
+---------
+
+- your transport encryption will only use self-signed TLS certificates,
+  which are vulnerable against MITM attacks.
+  the chatmail core's end-to-end encryption should suffice in most scenarios though.
+
+- your messages will not be DKIM-signed;
+  experimentally, most chatmail relays accept non-DKIM-signed messages from IPv4-only relays,
+  but some relays might not accept messages from yours.
+