Email Vault is a program that works with Mozilla Mail client to store your mails into a relational database (Postgres) and file your mails under multiple categories instead of folders (without duplicating the message).
Incidentally, some months ago I did conceive an idea to create an email consolidator and scribbled some notes on my trusty old sketch pad. Guess what I called it? Mail Vault!! I never really thought about using an SQL database, but there was a requirement to have a super-efficient backend mail store (not excluding the possibility of an SQL DB). The main goal Mail Vault is to import all your mails from various sources (mbox on a unix account, yahoo mail, outlook express mails, etc.), at the same time removing the duplicates. It would also have a super-intuitive interface (yet to be defined that allows you to perform complex query on your mails. But I didn’t get round to persuing it.
Later on, I found Zoe, which sort of does what I wanted to do. I tried to use it but it didn’t work out. Maybe I should give it another try.