Exchange 2003 Migration Wizard (can I allow users to access store while importing email?)

I will be using the Exchange Migration Wizard to migrate about 30 IMAP email boxes (20 GB) into Exchange 2003.  From my initial tests, it looks like the import will take 4-8 hours.  My plan is to block email coming into the server during the migration (basically hold it at a 3rd party "backup" mailserver).  While the Migration Wizard is importing the IMAP emails into Exchange, I want to migrate clients.  Is this o?.  My initial tests look like it works. I was accessing one mail box while importing a different one.  But I havent' tested accessing a mailbox while I was importing it.  Can anyone offer any advice on how to best manage importing email and migrating client simultaneously?

Obviously make the server multitask will mean a slower import, but I think that configuring clients for Exchange (mostly RPC/HTTPS) will help to keep users happy.  

I would appreciate if anyone can relate their experience with this and the best way to minimize the impact to a business of moving to Exchange.

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I have used exmerge successfully to do this, while users are logged in.
I know this has no relevenace to what you are using, but you can check this by moving 1 account in and being logged into that account while the data moves back in.
I would move the users in groups, wait for a group to finish, and then migrate the clients in that group while another is being moved.  Though I have gotten away with it for the most part, I have seen where users mail behaves erratic, and can corrupt, when trying to use it during migration.  Microsoft has always recommended disabling users to access mail while you are performing a migration, and though it could potentially cause corruption, I have never seen really seen any explanation as to how or why...just don't do it!  Exmerge can be a nice tool for this but it is easy to get in trouble with Exmerge and live users in the system.  Users tend to be deleting things and then Exmerge will put it back in which annoys users since they have to go back and mark those messages for deletion again.  Best policy?  Just get them out of the system for a few hours while you do the migration!  Users can  live without email for a few hours in order to keep a "non-corrupted" version of their Exchange system!  It's the least they can do to help you out with all the work YOU do!

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fitpcAuthor Commented:
Thanks for the comments guys,

Overall, I did what antioed suggested. Using groups. Although the way I did it, I ended up having to do it twice in order to import some subfolders (deleted the messages in the IMAP inbox so they woudn't come in twice and reran with just the required subfolders in the IMAP box).  In general most users wouldn't have been accessign their mailbox while I imported.  For reference though, I found the process to be pretty good and it reminded me a lot of Exmerge.  While I had used this before on SBS migrations it was the first time with IMAP folders with users hitting the system (albeit not much).

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