I have an exchange server 2010 with 2 databases and a exchange server 2003. The servers have been running for a while with out issues. I have begun to move users from and existing 2003 server. all users I placed in DB 1 work find, all users placed in DB 2 does not. Both DB's are mounted and display that they are healthy. Users in DB2 outlook says it is connected but unable to send or receive mail. No errors were reported failed or completed /w errors.
Would tend to suggest a database problem, so as you can have multiple databases I would just create a new one, move some test users to the new database. If they work, trash the old one.