Did you add this as a new domain in the Admin UI? Can you actually login with the credentials
myname@sub.mydomain.com and the password? The error message implies that the user isn't known. What does the following show (when run on your Zimbra server):
Code:
cat /etc/hosts
cat /etc/resolv.conf
dig sub.domain.com mx
dig sub.somain.com any
host `hostname` <-- use that exact command with bactticks not single quotes