I've had good success using mbsync to back up a variety of IMAP provided email services. It's not perfect, and does require a little bit of fiddling, but once i

I've had good success using mbsync to back up a variety of IMAP provided email services. It's not perfect, and does require a little bit of fiddling, but once it's set up, it Just Works™ the way it should. I've even used it to move ~300k mails from Google to Fastmail.