Our small shop has been on Exchange since forever (currently on 2003). However, we use a 3rd party managed Exchange provider and set tight limits on how much server-side storage we allow each user. The result is that our users all have additional email in local PST files, which we make daily backups of.
We're trying to move everyone to dovecot backed by Maildir. I basically have the reverse of this guy's question: http://serverfault.com/questions/8415/recommended-dovecot-thunderbird-to-exchange-outlook-migration-strategy , with the additional complication that Exchange doesn't currently hold all of the email, or even close to it.
For most of the users we can simply create the IMAP account in Outlook and copy/paste or drag the folder in Outlook, and Outlook pushes the messages up through IMAP to dovecot and they pop out the other side in the user's maildir.
For a couple of the users, though, we are stuck. They have multiple, large PST archives. Everyone is running Outlook 2003. Outlook 2003 supports two types of PST files: a backwards-compatible version that has a maximum size of 2GB, and a breaking-change one that supports several TB I believe.
We have users that have a mix of old format files that are at maximum size and new format files. We tried the same kludgy approach with them (its only two or three users that have this problem, and we only have a dozen email accounts or so total), but when you create a new IMAP-based account in Outlook 2003, it doesn't give you the choice of PST filetype, you get stuck with the 2GB max size.
The result is that Outlook eventually won't let you copy any more data into the PST, means you can't move any more of the data in the other PSTs up to the server via this method.
Creating a NEW IMAP account to the server with another 2GB PST won't work, because we can't seem to get Outlook to reproduce the IMAP-side folder structure without downloading all the messages, which just fills it back up to 2GB.
Tools like imapsync won't work because the email doesn't exist on the Exchange server, its only in the users' local PSTs.
I'm sorry if I'm missing something obvious here, I don't really have much experience with Outlook/Exchange.