NAT rules for local and distant mail server

RouterOS Beginner Basics
1

Migrated to MikroTik hEX.
Problem is to set up correct NAT rules for internal AND external mail servers.

  1. we are using Gmail with mail clients (i.e. Thunderbird - POP3 995; SMTP 465) plus some other servers/clients with POP 993
  2. we also have an internal email server on LAN 192.168.1.x and file sending via SMTP relay
  3. as far as we have internal server with services we want to use from inside and from outside, we have created rules for Hairpin 2 NAT. Still I think this rule should not make a problem for mail server. Am I right?
add action=masquerade chain=srcnat comment="Local to local NAT" dst-address=192.168.1.0/24 out-interface=bridge src-address=192.168.1.0/24
add action=dst-nat chain=dstnat comment="Hairpin to 2" dst-address=!192.168.1.0/24 dst-address-type=local dst-port=yyyy-yyyyy protocol=tcp to-addresses=192.168.1.X

At the moment SMTP rule looks like

add action=dst-nat chain=dstnat comment="Mail SMTP" disabled=no dst-port=25,587 protocol=tcp to-addresses=192.168.1.X

The problem is that sending mail via LAN SMTP server it comes back with

Diagnostic-Code: X-Postfix; mail for smtp.zz.com:25 loops back to myself

What would be correct rules to get working both email servers?

2

It looks more like problem with mailserver configuration to me. I don’t know Postfix, but based on the message, I’d say that server is sending mail back to itself (via the public address I assume). It shouldn’t happen at all, because the server should be aware that it handles target domain, and should not try to send mail for this domain anywhere else.