I've had this happen a few times when customers have more than
one mail domain or have done their testing with a "localhost" domain.
They change the primary domain and run
into problems. Don't panic - these are simple to fix.
The immediate problem is that you can't log back in to the
Administration Console once you've logged out. Actually you can,
but it's easy to miss what's going on.
Let's say that you used "localhost" while testing, and of course
that was set as primary. You've been logging into the Administration
Console as "Admin" and all is good. Now you have completed your tests
and have created your real domain: foo.com. You set that as primary,
do a little more work, log out and go home. The next day you
fire up the Admin Console, type in "Admin" and.. you can't log in.
The reason is that you changed the primary domain and you have
no Admin user there (or it has a different password). What you
need to do is log in as "Admin@localhost". You can then add an
administrative user to the "foo.com" domain so that you can use
the Administration Console without specifying "@localhost".
The primary domain never requires "@domain", but all others do.
Remember that and you won't be confused.