I receive my own email on a Mac running OS X Server 10.8, and just resolved a problem related to the creation of a second email user on that machine.
Let’s call the domain on which I receive mail on that machine, mydomain.com, which is completely different than the machine’s host name, which we’ll call, mymac.com. (The machine knows to receive email on mydomain.com since I added that domain in the Mail settings using the Server.app.)
This afternoon, I wanted to move my wife’s email to that machine as well.
Using Server.app, I created a new “services only” user, user2, and then setup her Mail.app with a new IMAP account pointing to [email protected]. That worked fine; Mail.app could login, and successfully manage IMAP folders.
The problem — The problem I ran into is that the Mac would not receive email on the address, [email protected]; it rejected all incoming messages with the error, “User unknown in virtual alias table”.
The solution — Thanks to this artice, I discovered that to setup a new mail account in OS X Server, it’s not enough to just create the user account, user2. You also have to right-click on the user name in the Users panel, and access the Advanced Options… where you’ll find an Aliases field, in which you have to enter the specific email address(es) on which you want that user to receive email. In my case, this was [email protected].
With that in place, the Mac then successfully began receiving mail.