PUBLIC MARKS from camel with tag user

qmail-smtpd executes as a qmail dedicated user, communicating on the external SMTP side using a socket connection, and talking to the internal processing section using a pipe. This architecture is very secure, as qmail-smtpd is allowed to read/write only what it owns (practically nothing), and, the eventual break of qmail-smtpd by a cracker would have a minimal impact on the safety of the system.