We've just done an Exchange 5.5 to 2003 migration. It was all set in motion before I arrived and for some reason the ADC (Active Directory Connector for Exchange 5.5) wasn't used.
The guy who did the migration over the weekend isn't in this week. He timed that very well as I've been dealing with the fall out.
I had a call yesterday from a user saying that my "From name" is wrong (the user, female, had got married since joining the company). It had to be picking it up from Active Directory, and as the user had no Display Name, Exchange was picking her name up from old Active Directory information. Giving her the right Display Name (in ADUC) solved that problem.
Had another today. A guy couldn't connect via POP - his authentication failed. I went into System Manager -> Servers -> <Server Name> , right click and "Properties" and to the "Diagnostics Logging" tab. I Chose "POP3Svc" from the left and set the logging level of all the right hand side entries to "Medium". Now Exchange started logging the guys login attempts.
One such log messages was: "Authentication attempt from [IP address] to [email address] has failed with error 0x52e" . A search through Google Groups lead me to a posting on the 19 July from the SBS (Small Business Server) group that pointed in the direction of this knowledge article: http://support.microsoft.com/default.aspx?scid=kb;en-us;296387 . That was the clue that I needed; I had the guy try logging on with various usernames as I kept making changes in Active Directory. What eventually worked was changing the mailbox alias to his short-form username which then matched his pre-Windows 2000 username.