@abcdw good clarifications for anybody reading, irrespective of the back-up question.
For conversations (properly) using PGP encryption, you anyway have to "process" e-mail (bodies) locally. How is PGP integration in 'notmuch' (and 'gnus')?
@tetrislife Idk about gnus, but it works flawlessly in notmuch.
The only thing is that you have to make additional configuration if you want to index encrypted emails.
@tetrislife using notmuch means that one have to process and index emails locally, so every notmuch user has his emails fetched locally at least or more likely copied from server.
Backing up emails doesn't help you with logins or other similiar thing, to be sure you can always use it for authentication you need to have an access (full control) to domain name of your mail address.