During a recent deployment we ran into an issue that had us scratching our heads. The deployment was smooth (once we took care of the 2.4GB mailboxes the executives had), users were happily sending and receiving mail, and all looked great.

This was a simple migration - an Exchange 2003 environment where we added an Exchange 2007 server, and configured it so that mail would route out of the Exchange 2003 environment into the Exchange 2007 server and out to the Internet.

But then we started getting calls from the helpdesk that some users intermittently were able to receive mail, but mail was being held in the outbox. Oddly enough, these calls were from users who had not yet being migrated off of Exchange 2003.