Send mail to email@example.com to subscribe. There is also a web archive and a nntp interface.
Most development activity occurs (still) on the qpsmtpd list, but we have a separate list for commit messages and such. It's called qpsmtpd-dev. Web archive and a nntp interface.
You can also subscribe to release notifications over at the Freshmeat page for qpsmtpd.