I have email going back more than 25 years from various software; in most cases an export dump when I stopped using, say, Pegasus Mail or Eudora. Since I started with Thunderbird I've created a new profile each year, and have profiles going back to 2008.

Earlier this year I imported all this historical data into the Mail Server on our Synology NAS. I don't use this server for current mail, but I can now search these old messages easily from within Thunderbird.

One thing I notice is how much more personal email there was before we started using chat services for group conversations. Nonetheless it's great being able to go back and read correspondence with, say, my dad who died in 2010.