[e12e]

I'd have a look at dbmail.org (an Imap server with a rdbms backend). If I've understood correctly, after some years they landed on a fairly straightforward schema with appropriate indexes.

There's also "Archiveopteryx": http://aox.org/

Another question is what do you want to do? I've not played that much with sql store for mail - but notmuch (and mailutils) make good use of Xapian for quick index/search of mail, if that is your end goal.