Setup Secondary Mail Relay

[kilburn]

Hmmm maybe I misunderstood you... The ispcp server is the backup or main one?
If it's the main one then I was correct, you don't need to do anything special.
If it's the backup server, you should manually remove/comment the domain in the files /etc/postfix/ispcp/domains and /etc/ispcp/postfix/working/domains . After that, postmap the former file and tell postfix to reload it's config. Voilà, the server will continue accepting mails for this domain, but they will be relayed to the main server instead of locally delivered