These patches to qmail and checkpassword allow you to store user mailbox information, address aliasing rules and virtual host configuration in a MySQL database. You can use them to set up a "POP toaster" - supporting large numbers of users with only one system account - or allow users to access their mailboxes with separate passwords from their system accounts.
A separate patch is available for netqmail. It is simply a version of the standard qmail patch tweaked to apply cleanly to a prepatched netqmail source tree.
The primary design goals for the qmail/MySQL project were secure, reliable code and maximum consistency with standard qmail semantics. A patched version of qmail should behave exactly as an unpatched installation would if no entries are written to the database, and in most cases you can mimic standard qmail behaviour with intuitive database configuration.
This project is currently not in development.
Development may continue in the future. I will answer requests for support as best I can. Nonetheless I must stress that this project is aimed at people with a good working knowledge of both qmail and MySQL. You do not need to be a qmail hacker or an SQL guru to use these patches but if there is any part of the documentation you don't understand, you are advised to read up on standard qmail and basic MySQL stuff before diving in.