If you want a list of good reasons to NOT use a database for email, have a look at http://www.memoryhole.net/~kyle/databaseemail.html
The funny part is at scale, all the reasons he has against mail in a db are the exclusive list of why I think its a good idea, not to mention it _is_ being done reliable for long enough to prove so.
I'd say ymmv, but I think I'd have to say _all_ our mileage varies from his:)