How to disable pop3 and imap access during the data migration process (duration 5 minutes to 10 minutes). Can i send a custom error message to our users ?
I don't recall the exact details, but I think both options are possible if you check the dovecot wiki. There is a good study regarding the proxy stuff, and some notes on sending custom errors back (along with a note that it's not supported on many clients)
If you use SQL then you could simply change the query temporarily
Also when I do my migration I change the permissions on the home folders first (root:root say) - this way if I muck something up they don't get logged into the old mailbox. Then I reset the permissions back on the new server and update the proxy settings. This also gives a random error back to the client in the meantime if they try and log in.
I should think that the above could even be scripted and run per mailbox for a very subtle downtime period... I use rsync once while the mailbox is online, then change perms so that the user can't login, then do a final rsync to get any changes across. I didn't check whether there is a problem with an existing connection still accessing the data after the perms were changed though...
Good luck
Ed W