Comment by ratww

I've used this project, but it's been a while: https://github.com/joeyates/imap-backup . It's a CLI app, though, so it's maybe not the best solution for non-technical end-users.

Some email providers have IMAP import, where you just give them the password and they'll do it for you. Not the best solution in terms of security but might be ok if you're getting rid of your account anyway.