You are making things uselessly complicated. Even absurd.
If I understand correctly, you use an encrypted email provider, because you want to encrypt your email. But you chose to store your mails unencrypted, because in order to encrypt them, you would have to accept not to have a recovery method.
Do you want, or don’t you want encryption ?
Encryption has a price. That price is reduced convenience, and more attention from the user. There is no such thing as a free lunch.
If the provider you have chosen tells you you can’t have a recovery method and encrypted emails, then you need to believe them. They are the experts, not you. If you don’t trust them with your security, then why did you select them in the first place ?
Now you’re imagining some horribly complicated setup:
- Pay for an encrypted email provider (Posteo is not free).
- Deactivate encryption at rest, because… something.
- Erase your emails from the server, negating the interest of buying an account in the first place.
- Elaborate a complex scheme by which you would download, then upload, said emails, to another provider… but… wait…
- You would still need to encrypt them on that cloud !
- But would that cloud service give you a method of “recovery” ? Doubtful ! You don’t know ! More problems ! Kicking the can down the road !
If you think that Posteo’s servers could “burn” or “corrupt” your emails, then that mysterious cloud provider (Google ?) could “burn” them as well, couldn’t it ?
Just stick with the email provider you choose, encrypt at rest, because otherwise, why not stay with Google ?, and at most, download regularly your emails on your own computer, to make a local backup (or several ones).
There’s no need to re-upload them somewhere else. One online backup (on Posteo) is more than enough, and then, yes, it’s reasonable to have a local copy as well (or several : you do make regular, automated images of your whole computer, don’t you ?).
Posteo offers POP and IMAP. So take advantage of it to access it through an email client, and you’ll have an automated local copy.
Since you (of course) have an automated backup procedure in place for your whole PC, that copy will be duplicated down the line. At which point, you will have a very thorough (but not perfect) protection.
There’s no such thing as perfect security. You still need not to loose your passwords, etc.