Did you ever disable a Active Directory Account which also disables the Exchange Mailbox on the older Exchange servers (Exchange 2000, Exchange 2003 with pre-SP2)?
You want to just disable the account, but not the mailbox. right? There is simple solution, read more below.
1. Open the disabled (and enabled) Active Directory account properties.
2. Go to Exchange Advanced and click "Mailbox Rights…"
3. Enable "Full mailbox access" to the "Self" account. If Self account is not showing up, click Add and type "Self" and click OK. (You could also check this as a "Associated External Account")
4. You are done. Mailbox should work even if account is disabled.
The idea here is inform Exchange that this account has a msExchMasterAccountSID by adding a SELF account with full mailbox permissions.
There is a HOT FIX available to fix the issue. Visit here for details: A hotfix is available to change the way that Exchange Server 2003 SP2 handles a disabled Active Directory user account that is associated with an Exchange Server 2003 mailbox
If you are still running with Exchange 2003 SP1 for strange reasons, Visit here: 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 (SP1) mailbox
More information at Technet blog: Exchange 2003 and disabled user accounts