This is a question for a qmail forum/mailing list 8)
Go to /var/qmail/control on your old server and edit the smtproutes file.
In this file, put one domain per line with the ZCS server the mail should go to:
Code:
xyz.com:zimbra.domain.tld
abc.com:zimbra.domain.tld
Then restart qmail.
However, a correct MX setup would be better...