Recommended Dovecot / Thunderbird to Exchange / Outlook Migration Strategy

We are moving ~60 user mailboxes from a Postfix/Dovecot/Maildir setup into Exchange 2007. I am aware of Microsoft's Transporter Suite, which will allow for an IMAP-to-Exchange conversion of mailboxes in bulk. However, there is a concern about how to clean up a botched process, should there be an unforeseen issue. Apparently, attempting to remove a corrupted Exchange mailbox for a user also results in a rather scary-sounding dialog that implies the user account will be removed as well, not just the corrupted box itself. While the original email store (in Maildir format) is available, we would need the ability to remove the bad box(es) in question and repeat the process, so this method is not a viable option.

I have grave concerns at this point about affecting a batch- or bulk-style transition without massive manual intervention. We are trying to avoid visiting 50+ desktops and manually moving email by using Outlook or Thunderbird as a "pivot" agent; while this is 100% guaranteed to work, it will take weeks or months to move over 2.5 million messages one account at a time.

So I have begun to look into other tools and methods, both open-source and commercial.

The first one I tried was OfflineIMAP, which as it turns out, is not very Exchange-friendly; a subtle effect of the migration process results in exhaustion of Exchange's named properties due to a unique header being generated for each email moved. A patch is available for this that changes the unique header into a single, generic header that has a unique identifier after it, avoiding the exhaustion issue. Yet, after applying the patch, there are still some issues. A pity, really, as it would have been nice to keep them in sync.

On the commercial side, I took a brief look at Transend Migrator in a trial mode. The results were mixed and there appears to be little documentation delivered with the binary. This is hardly encouraging, especially for something that end-users will readily notice if it goes horribly wrong.

There are other issues as well. The mailboxes are stored as Maildir, but the box names conform to Thunderbird's expectations (we've been on TBird for several years). Many box names do not match (Sent vs. Sent Items, Trash vs. Deleted Items, etc.) and will need to be translated effectively as they are ported. Yes, we pamper our end-users that much - a successful migration will consist of informing them we are switching to Outlook and giving everyone a small 30 minute class, followed by people openning their new Exchange-based mailboxes for the first time.

Someone, somewhere surely has something that works...

EDIT: A follow-up

The larch script, written in Ruby, has provided the easiest solution, although with a few caveats. Here's a heavily condensed version of what happens:

1. Obtain the user's username and password. Have the password reset after the transition.
2. Connect to the user's box via IMAP and ensure that the INBOX is subscribed, and that all messages are marked as read. There is an issue on the Exchange side that can prevent messages from successfully importing if the message is unread and in a certain state...
3. Change the postfix transport map to point to the exchange server, which causes all new deliveries to arrive at the Exchange server instead of the current email server.
4. Use the larch command to move the email over, omitting the user's Trash can. We had to capitulate and omit this because there were users turning the Trash into a personal filing system.
5. Note any messages that didn't transition. Re-examine and attempt to recopy them again. This will only move the newer messages.
6. Create a record in MySQL that instructs Dovecot to go into proxy mode for that user, and point the record at the Exchange server. This allows existing clients to connect unchanged until we are ready to set up their Outlook install.
7. Re-connect to the user's Inbox and check to see that all messages are copied.