How can I configure Postfix to ignore relayhost when forwarding mail?

How can I configure Postfix to ignore relayhost when forwarding mail?

I currently use the relayhost to send all outgoing email via an external SMTP service :

# /etc/postfix/main.cf
relayhost = [smtp.mandrillapp.com]

A couple of my domains are configured to send via an alternative SMTP service :

# /etc/postfix/relayhost_maps

@domain1            [email-smtp.us-east-1.amazonaws.com]
@domain2            [email-smtp.us-east-1.amazonaws.com]

However, a few of my customers have their incoming email forwarded to other accounts. I don't want to be sending forwarded email via my external SMTP service, instead I want it relayed directly by localhost.

For example, my machine accepts email for 'user@domain3.com', which the client has configured to be forwarded to 'other@hotmail.com'. What I'm looking for is a way to have any emails forwarded to 'other@hotmail.com' relayed directly by my server - and not relayed by my external SMTP service.

I think transport maps are close to what I need, and I have found lots of information on how to route to an external SMTP with transport mapping, but I can't figure how to relay from localhost only when forwarding mail.

I thought I had a solution here:

How can I configure Postfix to ignore relayhost for some domains?

... but when I tried it, I sent an email to myself and received it 2,500 times in some sort of loop between my machine and my SMTP gateway, and had to quickly stop Postfix!

So, basically I want to relay forwarded messages from localhost, and non-forwarded messages via various SMTP services.

Answer

Have a

transport_maps=hash:/etc/postfix/transport

line in main.cf, then add to /etc/postfix/transport :

other@hotmail.com smtp:

run postmap /etc/postfix/transport and reload postfix if you changed main.cf .