I'm not sure I like how hivemail does things. If I understand it right it uses a catch all address and puts all messages into the database. For large sites this would case a serious performance hit.
Postfix, Qmail, Courier IMAP, POP3 all have built in support for using a MySQL database for configurtation options. The mail gets stored on disk in the well tested and supported mbox and Maildir formats. One only needs to modify the auth daemon for IMAP to allow it to use VB's passwords.
Just my 2 cents.
-Kevin
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