Rewriting sender address based on the recipient

I'm trying to set up my postfix server to alter the sender address of outgoing e-mails for one specific recipient domain (example.com) -- otherwise I'd just configure smtp_generic_maps in main.cf and be done with it.

From what I understand, for this sort of configuration, there is a need to set up an extra transport that handles the traffic I want to get mangled.

So far I've set up an extra smtp transport:

transport:

example.com    drewrite:

master.cf:

smtp         unix  -       -       -       -       -       smtp
drewrite     unix  -       -       -       -       -       smtp 
    -o smtp_generic_maps=hash:/etc/postfix/rewrite_domain

rewrite_domain:

[email protected]     [email protected]

The problem is that amavis is also configured to run on that server, and it seems to hinder the address rewriting in some way I honestly can't wrap my head around.

The curious thing is that when I bypass the transport and set the smtp_generic_maps in main.cf to overwrite the sender address for all recipients; the address gets rewritten OK.

This led me to believe that perhaps the transport is not set up correctly; but postfix logs show that it is in fact being used (and when I did manage to bork it earlier I got deferred messages in the queue).

The only way I've been able to get this working was to disable amavis globally.