The POP protocol allows a client computer to retrieve e-mail from a pop server (remote server). The POP protocols are mostly mixed with IMAP.
POP3 is a client-server-based communication protocol that can be used to collect e-mails from an e-mail server. In addition to picking up, the log allows you to list the inbox and delete mail. The emails are actually processed offline on the client computer.
The abbreviation POP3 stands for Post Office Protocol Version 3 and designates a transmission protocol for retrieving e-mails from a server via an IP network.
The protocol works on the client-server principle. The incoming mail server is the POP server, the user’s e-mail program is the POP client.
The Post Office Protocol was already specified in 1984 in RFC 918. Other versions followed, such as POP2 in 1985 (RFC 937) and POP3 in 1988 (RFC 1081), which is still in use today.
The features of the communication protocol include the low level of complexity and the control of the transmission with text-based commands.
POP3 Working Principle
POP3 downloads the emails from the incoming mail server via an online connection. To do this, the client must establish a TCP connection to port 110. The server confirms the connection request with a positive message and initiates the authentication of the user with a user name and password.
After successful identification, the client can send commands to the server. If the client sends a quit command, the connection is closed again. After a quit command, the server executes other pending commands such as deleting emails in the inbox.
This prevents messages that have not yet been downloaded from being lost if the TCP connection breaks down during e-mail transmission.
After the connection has been reestablished, incompletely transmitted mails are still available because the server had not previously received a quit command from the client.
POP commands are made up of three to four characters and one or more parameters. The server answers the commands with positive or negative status reports and optional additional information.
Example commands are STAT for displaying the number of existing e-mails, RETR for retrieving e-mails or DELE for deleting an e-mail after the connection has ended.
