I use thunderbird connected to my normal mail account and the dovecot server to transfer mail archives between both.

I used to backup my mail to the “Local folders” account in thunderbird; that also works. It’s more for the convenience of having “everything on the server” and having access to my mail archive from a laptop and a desktop. It’s also easier to backup and restore for me.

Simplest solution is often the best. To add to this, Thunderbird has what are called Local folders. If you have your other email accounts added, you can simply drag and drop everything into the Local folder to move them off the cloud and into local storage. No email server required. Definitely a good idea to take backups if storing important stuff locally though.