I'm about ready to do the same 5.5 to 2003 migration myself. I have about 250 mailboxes on 5.5. I've setup a test lab twice and am running into the same error. It's a MAPI error when I try to move a mailbox from 5.5 to 2003 in the test environment.

Both test labs have been installed from scratch, so I'm stumped. Very agrivating as my 5.5 database is 15.5GB now. I'm almost at the database size limit ceiling. I'm sweating....
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let the wise listen and add to their learning,
and let the discerning get guidance- Proverbs 1:5