To give a little perspective, I basically don't have any pre-Yahoo/Google email history. So maybe going back 20 years or so and not much in the way of digital archives before about 10 years before that. Essentially nothing from BBS days, email on various proprietary systems, etc. Some shards here and there from the 1980s and a few things that were digitized prior.
I'm not sure if it's good or bad (probably a bit of both) that much of my early time online is largely gone--and I didn't have a significant time "online" before I was about 20 or so.
I have email from when I got my college email address in the late 90s, but I still can't find email from earlier, when I had a couple different addresses that I'd gotten from my local dial-up ISP. I was always -- even then, when hard drive space was expensive -- pretty good about keeping old data as I moved from machine to machine, so it really bums me out that I can't find that old stuff.
I have a collection of Eudora backups still kicking around (an old Mac OS9 and earlier vintage mail client). I don't have a _good_ way to read them any more (but at least I can grep them when I want to hunt an old email down... I've done that maybe twice in the last decade...)
For the 2000s, I do have an Outlook .pst somewhere I could probably find for work email. But I've probably pretty much exported any docs I care about from that era and likely wouldn't go to the trouble of digging anything else out.
I think I dumped a backup of my Juno emails (90s offline dial-up email for kids with parents too cheap to shell out for internet) but it's probably on some obsolete media like a Zip drive somewhere. Probably for the best.
Yeah, at the time I archived a bunch of MSN messenger chats, and looking back at those now is fantastic. Unfortunately it’s the only thing I have left. There must be a DVD somewhere with my my documents folder from 2004, but it’s lost to time.