What is a good way to maintain a backup of emails?

I want to be able to save my emails from various sources in a standard format and maintain a backup. An analog is taking pictures from multiple devices (e.g. cameras and smartphones) and saving all of them in a directory in JPEG format. The issue with archives (using export feature from KMail that I mainly use or any other client) is that there is no easy way to determine if an email has been lost between two backups. An analogy is taking backups using rsync. To explain, let us say I keep two backups, B1 and B2 where the former is updated from my data source (e.g. my smartphone) and B2 is synched from B1 after a few days. It is easy to spot accidental deletions by doing dry runs in verbose plus delete extraneous files in destination mode, when doing the B1 to B2 sync. Related is also the problem of introducing email duplicates. Is there a smart way to save all emails?