Commenting because I too am interested in this. I also would like to see if anyone has solved the issue with Thunderbird and Yahoo email where only 10,000 emails are kept. I have an inbox with about 20+years of emails, and with the 10k cap, I realized Thunderbird was literally deleting the oldest ones. Since I wisely told it not to delete them on the server when it copied them, I didn’t lose them, but with that volume of emails, I really don’t want to have to manually move them into folders. Guess this turned into a ramble, but it would be nice to have a backup since you never know when one day you will wake up to be told service x is shutting down.