Mail is stored in a maildir called maildirname, normally Maildir, in the user's home directory.
qmail-pop3d is normally invoked under qmail-popup, which reads a username and password, and /bin/checkpassword, which checks the password and sets up environment variables.
qmail-pop3d has a 20-minute idle timeout.
qmail-pop3d supports UIDL, TOP, and LAST.
qmail-pop3d appends an extra blank line to every message to work around serious bugs in certain clients.
qmail-pop3d is based on a program contributed by Russ Nelson.