Allow e-mails to be sent via FW

[Beez]

How to configure NAT to pass the mail traffic when you have 3 interfaces
1.LAN
2.DMZ
3.WAN

Mail is only working one way either send or receive but not both ways..

[GruensFroeschli]

Without giving any information about your setup (No, saying that you have 3 interfaces is NOT giving information) noone will be able to help you.

Start with the basic things like:
How is your network set up (diagram, inclusive the used IPs),
what firewallrules you have where (screenshots of the rules),
NAT-rules (screenshots of them),
how you test (where is the testclient,
to what do you connect),
what you expect,
and what actually happens.