The Post Office Protocol (POP) is one of the two most prevalent network protocols for e-mail retrieval, the other being IMAP. The POP protocol has been developed through several versions, with version 3 (POP3) being the current standard.
To test a POP3 server you can communicate manually with it through telnet:
telnet pop3.example.com 110
A sample POP3 session:
user johndoe pass secret quit
Testing an encrypted connection:
openssl s_client -connect pop3.example.com:995