My email provider ought to have its own backups for their own service-continuity. (IMAP in this case, but the picture looks very similar for POP.)

Meanwhile, my desktop e-mail client (Thunderbird) has its own local files which are captured by a consumer online-backup solution (e.g. Backblaze) along with other documents, photos, etc. That means both local-copies of server mail, and also items which have been perma-downloaded into mailboxes and purged from the server.

In a way, my biggest worry is not backup-coverage of the bytes per se, but potential archeological issues when it comes to arcane mailbox formats. (My second-biggest worry is that my organization is terrible and Inbox keeps growing.)