This is very true and yet the search tools supplied with these knowledge bases are terribly inadequate. Is there a better, standalone tool out there for turning email archives, from multiple sources, into a usable database with a powerful query language?

Not sure if that's what you're looking for, but you could use notmuch[0]. Haven't used it a lot, but I think it can import an mbox? Maybe you can merge multiple mboxes together?

[0]: https://notmuchmail.org

I looked into that a few times, just haven't had the patience to get through the setup process. Maybe I'll get it done this time. There goes my afternoon :)

7 hours later: Now I remember why I never got it running, it's an impossible task to accomplish.

I was keeping an eye on this in the hope that someone had a good answer for it. What features would you consider adequate for this kind of email search? At the moment I just have everything in Thunderbird and it does a passable full-text search on a ~5gb mailbox but I haven't experimented with anything fancier.