[strumptrumpet]

Is your reply bourne out of experience?

Mail hasn't changed much in more than a decade. Systems like Ubuntu update cleanly from release to release. Maildirs are very easily migratable.

You can literally set up a new mail host, and then rsync the old Maildir contents -- aka, all your user's e-mail -- onto your new host, whenever time permits, without disrupting mail delivery or reading. User's historic e-mail will simply gradually fill in.

If your VPS disappears, or hardware failure occurs, or user error strikes -- simply restore your Maildirs on a new system from your backups (tarsnap works great!).

This documentation is long and thorough, but once you know what you're doing, setting up a basic mail server, even manually, should only take you a few hours at the most.