This is a wierd problem. A user on Zimbra 6.0 Network Ed. sent an external email and got a auto reply (vacation) from this person with our zimbra servers FQDN.. This external e-mailaddress is not on our servers at all.
Our zimbra user --> firstname.lastname@example.org
Our zimbra user <--- auto-reply from email@example.com
The firstname.lastname@example.org address is not an account in our zimbra server at all.