High Quality Content by WIKIPEDIA articles! In computing, local e-mail clients can use the Kerberized Post Office Protocol (KPOP), an application-layer Internet standard protocol, to retrieve e-mail from a remote server over a TCP/IP connection. The KPOP protocol is based on the POP3 protocol with the differences that it adds Kerberos security and that it runs by default over TCP port number 1109 instead of 110. One mail server software implementation is found in the Cyrus IMAP server.