Thanks, all your comments are noted.
As I said, you can migrate users individually. You could easily do 10 users a day during coffee breaks etc and be done in a month plus. Do 40 a day and you're done in 10 days. The only time you'll burn is in the learning curve, not the actual mailbox migration which takes no time at all with POP accounts.
That's interesting, as I (wrongly) assumed switching from mbox to maildir was an all or nothing process. You're saying we can run half the mailboxes in mbox format and the other half in maildir format?
In which case we can get going with this sooner than I thought.
Always test with a dummy mailbox first to iron out any issues. Then start migrating the problem users first, the smart phone users who tie up their mailboxes for many minutes during download.
Thank you - I would probably start with the CEO's mailbox first and then go from there >:-D
