Originally Posted by Paul-Xensia

All the above is quite possible to do, until you get to point 6, multiple SMTP servers should be no problem, multiple POP3/IMAP boxes becomes a challenge because you need a way of splitting the data out, or sharing it across multiple boxes, this introduces lots of complexities.

If you ever end up with any large mailboxes, you will certainly feel the benefit of maildir over mbox.

We have implemented the above (minus point 6 and quotas) using all open source packages. Quota functionality should be possible to add, although there are some issues that need to be thought about..

All the software is freely available, it will either cost you time to implement it, or money to get someone else to do it for you..