@abcdw I do wonder if 'notmuch' (and 'gnus') users have more of a back-up ethic than webmail users. I don't know the pre-webmail era, but wasn't it there? E-mail back-up may not have been as big a deal then, since there were no online transactions or e-mail ID used for service log-ins.
@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.
@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.
https://a3nm.net/blog/notmuch-encrypted.html