Is the other server running Postfix as it's MTA? If so, you can add this to
/etc/postfix/transport:
Code:
my-zimbra-domain.com smtp:192.0.0.121
Then run this once it's saved:
Code:
postmap /etc/postfix/transport
I'm not personally familiar with any other mail servers except Postfix, but someone else should be able to help providing you supply us with the necessary information.