I have every email sent & received since '94, all in mbox format by year by email address. Easy to do and it comes in handy. Why not record conversations too!

I've also been archiving my emails since the early 90s. I've got a few hundred thousand messages. I've always used procmail to store in mbox format. I use shell/grep etc for searching. With procmail I archive like so;

$HOME/Mail/year/SenderOrRecipientAddr

where SenderOrRecipientAddr is either the senders email addr or the recipients, depending upon whether it's mail to me or from me. This way for example everything I send and receive to/from joe@smith.com is in the same mbox file.

And storing it under $HOME/Mail allows imap to serve it up.