Reliable ways to compile email history into an archive?

Over time it's common to use quite a number of emails, whether just personal and work addresses scaling up to something like a catch-all address on a personal domain.

I see a lot of great recommendations for email clients on HN (recently I've been looking at fastmail as a gmail alternative and the custom domain support seems like a great feature). But to my knowledge, it's difficult to bring all the accumulated backlog of email backups and unify them in one archive.

My imagined ideal solution is a tool that allows you to import all the different saved backups from old accounts, that will discover and deal with duplicates if you have overlapping backups, and which will allow you to query the archive to discover interesting patterns like volume of email sent/recieved or time between receiving and replying.

Are there tools out there that could help to ingest and organize all that email history, and potentially remain in sync with current accounts moving forward?