Configuring Postfix to Use Duplicate Services with Different Options

Question

I'm attempting to create two distinct configurations for sending outbound emails in Postfix, utilizing two binding IPs (primary=11.11.11.11 and secondary=22.22.22.22) for smtpd. However, I'm encountering issues with my current setup. Here's what I've tried so far:

1. Added the following line to main.cf:

    sender_dependent_default_transport_maps = hash:/etc/postfix/sender_transport smtp_bind_address = 11.11.11.11

2. Created and postmapped /etc/postfix/sender_transport with the content:

    @example.com smtp2

3. Added a line to master.cf for the second smtp as:

    smtp2 inet n - y - - smtpd
      -o smtp_bind_address=22.22.22.22

After restarting Postfix, I received this error:

Mar 23 09:02:23 test1 postfix/master[93837]: fatal: 127.0.0.1:smtp2: Servname not supported for ai_socktype

When I added the smtp2 service to /etc/services and tried sending from example.com, I encountered these messages:

warning: connect to transport private/smtp2: No such file or directory

Mar 23 09:02:53 test1 postfix/error[93181]: B8B8E3F1B4: to=foo@bar.example, relay=none, delay=564, delays=564/0.02/0/0.01, dsn=4.3.0, status=deferred (mail transport unavailable)

I suspect there might be a conceptual misunderstanding in my approach, but I'm not sure what it is.

Answer 1

There is no need to mingle outgoing port and internal reachability. The components to accept messages via smtp (smtpd = daemon) and the components to transmit messages via smtp (sans d = client role) are separate.

When you create a new outgoing SMTP transport, you want to bind it to a unix socket, only accessible to postfix. Such sockets are identified by (file)name, in no way connected to named port reservations in /etc/services.

After reading man 5 master, then man 8postfix smtp and man 5 transport you will likely settle with configuration like this:

# in master.cf:
smtp-identifier unix - - y - 3 smtp
 -o syslog_name=postfix/$service_name
 -o smtp_bind_address=192.0.2.22
 -o smtp_bind_address6=2001:db8::d34d:beef

# in any transport map:
#  a name before the colon overrides transport
#  a name without colon would merely override nexthop of unchanged transport
@example.com    smtp-identifier:

The TCP port that service eventually uses to transmit messages will be dynamically allocated.