You wouldn't have to do anything but train your users if their email clients allow logging onto two IMAP accounts at once: one on the old server, and one on the new.
That's not the goal.
There are 5K users and about 8 support staff total. User training is out of the question, beyond a pop-up window in the webmail client that says "your old mail is in the folder called X" where X is a folder pointing or proxied to the old server (UW IMAP).
Both pass off authentication to AD. Since the proxy approach doesn't look practical I'm leaning towards modifying the webmail client to re-use credentials and copy over messages to the new server.
--
Locusts and honey ... not since John The Baptist has there been a voice like that crying in the wilderness. ... Every man knows he is a sissy compared to Johnny Cash.
-- Bono
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