[edwinbalani]

You can also find the 'GitHub version' (which I'd guess is where distributions get their versions from) at https://github.com/imapsync/imapsync . I used this for an uncomplicated transfer from one IMAP server to another; it felt pretty slow but with the right CLI flags it is interruptible/restartable, and I didn't end up with lost or duplicate mail.

It comes with the author's custom "no limits" licence (looks a lot like the WTFPL), so it probably qualifies as free & open source, but you don't get a support contract of course.

The author's site also has a "give me your IMAP credentials and I'll migrate your mailbox for you, free up to a size limit" service, https://imapsync.lamiral.info/X/ , which boggles the mind. On principle I couldn't trust it.