A small application built to test the performance of a pop3 authentication system using a lot of concurrent connections. It can also be used to try lots of password against a pop3 server. It is capable of using up to 1024 sessions (or more using multiple processes). However with this amount it is capable of reducing internet connections to a crawl and also greatly increasing the load on the server.
You can download it from the following link: https://packetstormsecurity.com/files/download/108730/brute-pop3-2012-01-05.c
Source: https://packetstormsecurity.com/files/108730/POP3-Password-Brute-Forcer.html