One of the more common events we see in organizations that are migrating to Exchange 2003 from 5.5 is the 9548.
Event Type: Warning
Event Source: MSExchangeIS
Event Category: General
Event ID: 9548
Computer: Computer Name
Description: Disabled user /o= Organization Name /ou= Administrative Group Name /cn=Recipients/cn= Computer Name does not have a master account SID. Please use Active Directory MMC to set an active account as this user's master account.
This event occurs when you have a disabled user account that is not configured correctly. The mailbox may not be able to receive mail and the mailbox may not be able to be logged in to by certain users. In the past we had a utility called NoMas.exe that would fix these accounts, but it was a manual process to run it unless you set it up to run at a scheduled time.
The great news is that we have released a Post SP1 hotfix that should make administrators jobs a little easier. The Post SP2 hotfix is soon to be released.
903158 A hotfix is available to modify the way that Exchange Server 2003 handles a disabled Active Directory user account that is associated with an Exchange Server 2003 mailbox
You will need to call Microsoft Support to receive the hotfix.
(Kudos to Alex Seigler and others for helping to push this fix through...)
(Edit: The post SP2 version of this hasn't been released yet.)