added lmtp: prefix and documentation

core/admin/mailu/internal/views/postfix.py

@@ -36,15 +36,23 @@ def postfix_alias_map(alias):
 def postfix_transport(email):
     if email == '*' or re.match("(^|.*@)\[.*\]$", email):
         return flask.abort(404)
-    localpart, domain_name = models.Email.resolve_domain(email)
+    _, domain_name = models.Email.resolve_domain(email)
     relay = models.Relay.query.get(domain_name) or flask.abort(404)
     target = relay.smtp.lower()
     port = None
-    if use_mx := target.startswith('mx:'):
+    use_lmtp = False
+    use_mx = False
+    # strip prefixes mx: and lmtp:
+    if target.startswith('mx:'):
         target = target[3:]
+        use_mx = True
+    elif target.startswith('lmtp:'):
+        target = target[5:]
+        use_lmtp = True
+    # split host:port or [host]:port
     if target.startswith('['):
         if use_mx or ']' not in target:
-            # invalid target (mx: and [])
+            # invalid target (mx: and [] or missing ])
             flask.abort(400)
         host, rest = target[1:].split(']', 1)
         if rest.startswith(':'):
@@ -57,29 +65,38 @@ def postfix_transport(email):
             host, port = target.rsplit(':', 1)
         else:
             host = target
+    # default for empty host part is mx:domain
     if not host:
-        host = relay.name.lower()
-        use_mx = True
+        if not use_lmtp:
+            host = relay.name.lower()
+            use_mx = True
+        else:
+            # lmtp: needs a host part
+            flask.abort(400)
+    # detect ipv6 address or encode host
     if ':' in host:
         host = f'ipv6:{host}'
     else:
         try:
             host = idna.encode(host).decode('ascii')
         except idna.IDNAError:
-            # invalid target (fqdn not encodable)
+            # invalid host (fqdn not encodable)
             flask.abort(400)
+    # validate port
     if port is not None:
         try:
             port = int(port, 10)
-            if port == 25:
-                port = None
         except ValueError:
-            # invalid target (port should be numeric)
+            # invalid port (should be numeric)
             flask.abort(400)
-    if not use_mx:
+    # create transport
+    transport = 'lmtp' if use_lmtp else 'smtp'
+    # use [] when not using MX lookups or host is an ipv6 address
+    if host.startswith('ipv6:') or (not use_lmtp and not use_mx):
         host = f'[{host}]'
+    # create port suffix
     port = '' if port is None else f':{port}'
-    return flask.jsonify(f'smtp:{host}{port}')
+    return flask.jsonify(f'{transport}:{host}{port}')
 
 
 @internal.route("/postfix/recipient/map/<path:recipient>")

docs/webadministration.rst

@@ -215,22 +215,29 @@ On the new relayed domain page the following options can be entered for a new re
 * Relayed domain name. The domain name that is relayed. Email messages addressed to this domain (To: John@example.com), will be forwarded to this domain.
   No authentication is required.
 
-* Remote host (optional). The SMPT server that will be used for relaying the email message.
-  When this field is blank, the Mailu server will directly send the email message to the relayed domain.
-  As value can be entered either a hostname or IP address of the SMPT server. 
-  By default port 25 is used. To use a different port append ":port number" to the Remote Host. For example:
-  123.45.67.90:2525.
+* Remote host (optional). The host that will be used for relaying the email message.
+  When this field is blank, the Mailu server will directly send the email message to the mail server  of the relayed domain.
+  When a remote host is specified it can be prefixed by ``mx:`` or ``lmtp:`` and followed by a port number: ``:port``).
+
+  ================  =====================================  =========================
+  Remote host       Description                            postfix transport:nexthop
+  ================  =====================================  =========================
+  empty             use MX of relay domain                 smtp:domain
+  :port             use MX of relay domain and use port    smtp:domain:port
+  target            resolve A/AAAA of target               smtp:[target]
+  target:port       resolve A/AAAA of target and use port  smtp:[target]:port
+  mx:target         resolve MX of target                   smtp:target
+  mx:target:port    resolve MX of target and use port      smtp:target:port
+  lmtp:target       resolve A/AAAA of target               lmtp:target
+  lmtp:target:port  resolve A/AAAA of target and use port  lmtp:target:port
+  ================  =====================================  =========================
+
+  `target` can also be an IPv4 or IPv6 address (an IPv6 address must be enclosed in []: ``[2001:DB8::]``).
 
 * Comment. A text field where a comment can be entered to describe the entry.
 
 Changes are effective immediately after clicking the Save button.
 
-NOTE: Due to bug `1588`_ email messages fail to be relayed if no Remote Host is configured. 
-As a workaround the HOSTNAME or IP Address of the SMPT server of the relayed domain can be entered as Remote Host.
-Please note that no MX lookup is performed when entering a hostname as Remote Host. You can use the MX lookup on mxtoolbox.com to find the hostname and IP Address of the SMTP server.
-
-.. _`1588`: https://github.com/Mailu/Mailu/issues/1588
-
 Antispam
 --------