邮局协议（Post Office Protocol，简称POP）是TCP/IP协议族中的一员，由RFC 1939定义。本协议主要用于支持使用客户端远程管理在服务器上的电子邮件。最新版本为POP3，全名“Post Office Protocol - Version 3”，而提供了SSL加密的POP3协议被称为POP3S。

POP支持离线邮件处理。其具体过程是：邮件发送到服务器上，电子邮件客户端调用邮件客户机进程以连接服务器，并下载所有未阅读的电子邮件。这种离线访问模式是一种存储转发服务，将邮件从邮件服务器端送到个人终端机器上，一般是PC机或MAC。一旦邮件发送到PC机或MAC上，邮件服务器上的邮件将会被删除。但目前的POP3邮件服务器大都可以“只下载邮件，服务器端并不删除”，也就是改进的POP3协议。

In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP has been developed through several versions, with version 3 (POP3) being the last standard in common use before largely being made obsolete by the more advanced IMAP. In POP3, e-mails are downloaded from the server's inbox to your computer. E-mails are available when you are not connected.