Postfix Relay Based on Recepient

Question

What is the best practice when one has to implement this sort of ruleset in postfix:

• Email sent to [e-mail], [e-mail], etc ought to be relayed to smtp.a.example.com.
• Email sent to everyone else @example.com ought to be relayed to smtp.b.example.com.

Ideally this shouldn't depend on the content of the message - just the RCPT TO SMTP command.

Answer 1

You have to use transport_maps in your main.cf

#/etc/postfix/main.cf
#...
transport_maps = hash:/etc/postfix/transport
#...

#/etc/postfix/transport
[e-mail]    smtp:smtp.a.example.com:25
[e-mail]    smtp:smtp.a.example.com:25
example.com      smtp:smtp.b.example.com:25

---

transport_maps (default: empty)

Optional lookup tables with mappings from recipient address to (message delivery 
transport, next-hop destination). See transport(5) for details.

Specify zero or more "type:table" lookup tables. If you use this feature with 
local files, run "postmap /etc/postfix/transport" after making a change.

Ref: Postfix Docs