Here is my scenario... I have a single Exchange 2000 Enterprise Server in my Active Directory. The database was getting too big, so I added another server and moved a large portion of the users to this newer, faster server. I used the Exchange Tasks Wizard to move the mailboxes. All of this went extremely well, with one exception. People lost their abililty to "send as" in mailboxes that they used to be able to do this. For example, the facilities people had their own personal mailbox and a facilities mailbox in their Outlook client and used to be able to reply as "facilities" to emails received in that box. This no longer works after these mailboxes were moved.
Additional info: Server 1 is running Win2k Advanced Server with SP2 and Exchange 2000 Enterprise with SP2. Server 2 is running Win2k Advanced Server with SP3 and Exchange 2000 Enterprise with SP3. Anyone with Domain Admin rights still can "send as" but regular users lost the ability after the move.