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Hello.
Recently, i migrate my Exchange Server to Zimbra, and my migration occour without problems.
When an user send a message to my zimbra server and user no exist at accounts, user receives this error:
recipient address rejected: domain.com
Unable to deliver message to <user@domain.com>
Normally, user receives "User Unknown" or "User unknown in local recipient table".
Bugzilla
Wiki
Source
Message when User Unknown 
Linear Mode
