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We have a HUGE (over 100 gb) mailbox on Exchange 2003 that we need to migrate over to Exchange 2007. The problem with this mailbox is that it is a shared mailbox for over 100 users. At first we were just going to creat a RSG and restore from backup tape to 2007 but found out rather quickly that, that wasn't an option. We know that Ex-Merge is out due to size and we thought about just trying an in-place upgrade on a test exchange server from 2003 to 2007 with this mailbox restored on it from tape just to see what would happen although I know it isn't supported. This was setup originally to share project emails but has grown over the years to this. It has around 840,106 total items and is just over 99GB in size. We tried to archive some of it but the archive software left stubs which still counts as an item and the archiveing solution is so slow that users have refused to use it, plus it opens in a GUI, not integrated in outlook so they don't like that either. What would be the best possible way to go about doing this with the least amount of downtime, if any? Thank you in advance for your help.

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You can't do an in place upgrade from Exchange 2003 to 2007, one is 32 bit the other is 64. Its not about it not being supported, its not possible. – Sam Jan 6 '10 at 15:10

If you are going to be installing your new Exchange 2007 server on the same Forest and Network, then you should be able to use the Move-Mailbox utility in Exchange 2007 to migrate this mailbox from the Exchange 2003 Database to the Exchange 2007 one. Sure it will take quite some time to do, but it should work.

That being said, do you really just want to continue with this monolith of a mailbox as it is? Would you be better spending some time archiving, splitting the mail into separate mailboxes or just plain deleting some. It must be a nightmare to manage and use. If that's the case, it would be better to do any of this before you move.

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+1 - The mailbox size isn't relevant to the migration strategy. Move-Mailbox is "where it's at". The OP really needs to re-think the usage strategy of the mailbox, IMO. That mailbox is bigger than many Mailbox Store databases at my Customer sites. – Evan Anderson Jan 6 '10 at 16:08

I think your only option is to use move mailbox to move the mailbox to the new server. This should not incur any downtime as items can still come in. I would probably first move this mailbox to it's own storage group first.

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