SMTP stands for Simple Mail Transfer Protocol. It is used to transfer RFC822-style messages between different mail hosts as well as to submit new messages to a host for delivery. SMTP is in very wide use (it originated in 1982) and is defined by RFC821.
POP3 stands for Post Office Protocol version 3. POP3 is a very limited protocol for accessing a single mailbox. It is much less capable than IMAP. POP3 is very widely used and is defined by RFC1939.
IMAP stands for Internet Message Access Protocol. It is a method of accessing electronic mail messages stored on a (possibly shared) mail server. In other words, it permits a “client” email program to access remote message stores as if they were local. IMAP is defined by RFC2060.
这三种协议可以分为两类：SMTP是一类；POP3和IMAP是一类。
简单理解：SMTP用于服务器之间的相互通信或者客户端往服务器发送邮件；POP3和IMAP用于客户端从服务器拉取邮件。
POP3跟IMAP的区别：前者很古老，功能也没有后者强大。
看图吧，更直观一些：
比如：张三的邮件地址是
[email protected]
，李四的邮件地址是
[email protected]
；张三想给李四发邮件。
第一步：张三用他的邮件客户端把邮件推送到 outlook 的邮件服务器上，这里用的是SMPT协议；
第二步：outlook邮件服务器将张三的邮件转发到163的邮件服务器上，这里用的是SMTP协议；
第三步：李四用他的邮件客户端从163的邮件服务器上把邮件拉取下来，这里用的是POP3或者IMAP协议。
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