The qmail Handbook

qmail-qfilter is a utility that allows messages to be filtered before they're passed to qmail-queue. Using qmail-qfilter, programs and scripts can be used to alter messages (add or adjust header fields, for example) or refuse unwanted messages (such as those with executable attachments).

Bruce Guenter wrote qmail-qfilter and maintains a Web page for it (http://untroubled.org/qmail-qfilter/).