This requires Postfix version 3.
/etc/postfix/main.cf:
transport_maps = hash:/etc/postfix/transport
/etc/postfix/transport:
example.com smtp:example.com:123,example.com:234
NOT: smtp:example.com:123, smtp:example.com:234.
Run postmap hash:/etc/postfix/transport after editing the file.
Instead of hash use whatever is suitable.
Wietse
