In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a mail server. Today, POP version 3 (POP3) is the most commonly used version. Together with IMAP, it is one of the most common protocols for email retrieval.
RFC 1081 – Post Office Protocol – Version 3
RFC 2195 – IMAP/POP AUTHorize Extension for Simple Challenge/Response
RFC 2595 – Using TLS with IMAP, POP3 and ACAP
RFC 8314 – Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access