SMTP protocol belongs to TCP/IP protocol family, which helps each computer find the next destination when sending or transmitting letters.
POP post office protocol is responsible for retrieving e-mail from the mail server. POP3 is the first offline protocol standard for Internet e-mail.
Internet Information Access Protocol (IMAP) is a new protocol superior to POP. Like POP, IMAP can download mail, delete mail from the server or ask if there is any new mail, but IMAP overcomes some shortcomings of POP.
Extended data
The advantage of post office protocol (POP) is that it supports mobile computing. Because the user's letters are stored in the POP server, rather than directly and automatically stored in the user's computer, the user does not need to establish a permanent connection with the "post office" to wait for the mail that may be sent at any time.
This makes it easy for those desktop PCs or notebook computers that are not easy to directly connect to the LAN to get mail services through dial-up Internet (establishing temporary TCP/IP connection), so that it has become the main form for scattered Internet users to get mail services at present.
Baidu Encyclopedia-Email Protocol