I don’t really understand the urge to archive mail to a separate program. Backup, absolutely. But the mail client is itself the definitive archive of the mail. I have dumped some eudora mailboxes into eaglefiler, but it was never an urgency even if I don’t want to boot up snowie to run eudora.
Once upon a time I used to at least try to keep mail cleaned up and organized. But disk space got cheap, search got fast, and time got expensive so I stopped fiddling with it. There’s no real point in deleting things (though I usually dump out server status messages once a year and spam more often). I use mail rules to sort mailing lists and such into their own folders and I’ve greatly reduced the number of folders that I move things to manually.
Mailing lists especially I prefer to keep. There are a bunch of them that I don’t read so much as search for particular things. E.g, many or most microscopy lists are taken over by gear restoration people who crowd out any talk about what people are actually looking at (if anything). But search can turn up the good stuff, and every now and again even the gearheads are useful. Searching directly on mac is faster and much more effective and flexible (regular expressions via bbedit if needed) than search on associated online forum archives.
Mostly I use the fastmail.com web interface, but I have an instance of Mail.app on an older mini running to back that up and move older mail off line (by hand). I do have old old mail repositories from the early days (pre personal computers) that I could probably merge in with the others, but I’ve never used a mail program that didn’t save as plain text files, so even that’s still all easily available with a search, usually in houdah spot.