For the webmail provider, Qmail was used for all inbound/outgoing e-mail. Some modifications we made was to replace the delivery process with one that looked up the mail storage backend a given user was on and passed on the mail to qmails normal delivery process.

We also modified the delivery process to embed additional information in the message file names so that we could get away with just reading the directory to get file size (instead of additional stat() calls for each file), flag status etc.

Eventually we added quota checking and a cache of parsed header information (coupled with a custom command added to the POP server to list that info).

Qmail was ideal for that given that it consists of a bunch of small, easily understood components that all are documented extremely well and can be tested on the command line.

For the queueing system, any mail system works fine if your requirements does not include absolute ordering. All we needed to do was poll a Maildir (or POP server) to handle incoming messages, or inject outbound e-mail for outbound messages. It was fast and simple at a time when there was a distinct lack of open source dedicated queueing implementations.