Post Office Protocol

Post Office Protocol (POP)

Acronyms Technology

Short for Post Office Protocol according to AbbreviationFinder, POP is a protocol by which the mail is received (the one that sends the received mail from the server to the computer). It should be understood, however, that the POP and IMAP protocols are only used for communication from the server to the user’s PC, within the provider’s private network, since the server receives the mail in turn, from another server to over the Internet, but using SMTP as the protocol between both servers.


The meaning of the acronym POP is Post Office Protocol. Unlike other previously created protocols such as SMTP, POP does not need a permanent connection to the Internet, since it is at the time of connection that it requests the server to send the correspondence stored on the server for said user.


An alternative protocol to POP is IMAP (Internet Message Access Protocol), with which it is possible to examine the mail directly from the server in the same way as if it were downloaded to our PC. In addition, it allows you to locally delete received messages by keeping them on the server.


If you are permanently connected to the Internet, the mail client programs can be configured in such a way that the request to the mail server is made automatically from time to time and in this way warns the user that he has mail pending receipt.

The current situation is that the SMTP protocol is used for sending mail and the POP protocol is used for receiving mail, but now in its third version since its appearance, POP3.

Most POP mail clients are automatically configured to delete the message on the mail server after it has been successfully transferred, however this setting can be changed.


POP is fully compliant with major Internet messaging standards, such as Multipurpose Internet Mail Extensions (MIME), which allow mail attachments. POP works best for users who have a system in which to read mail. It also works well for users who do not have a permanent connection to the Internet or to the network that contains the mail server. Unfortunately for those with slow connections, POP requires client programs to download the full content of each message after authentication.


The latest version of the POP standard protocol is POP3. However, there are also a variety of variants of the POP protocol that are not as popular:

  • APOP – POP3 with MDS authentication. In this protocol, the mail client sends a hash

encryption of the password to the server instead of sending an encrypted password.

  • RPOP – POP3 with RPOP authentication, which uses a user ID similar to a password to authenticate POP requests. However, this ID is not encrypted so RPOP is not more secure than the POP standard.

Preparing a POP server

A POP toaster, or dedicated POP server receives mail for authorized users and allows them to collect it over the network. A server with these characteristics has three essential components:

  • Storage boxes: storing incoming messages on disk.
  • Mailbox service: allowing users to collect mail over the network.
  • Account management: setting passwords for authorized users.
  • Common options supported by sendmail.
  • Mailbox storage: mbox format in / var / spool / mail / $ USER
  • Mailbox service: qpopper or ipop3d. (Note that all versions of qpopper prior to 2.51 allow remote users to get hold of your machine.)
  • Account management: UNIX standard accounts in / etc / passwd.


To establish a connection to a POP server, the mail client opens a TCP connection on port 110 of the server. When the connection is established, the POP server sends an invitation to the POP client, and then the two machines send each other other commands and responses that are specified in the protocol. As part of this communication, the POP client is asked to authenticate (Authentication Status), where the username and password of the user are sent to the POP server.

If the authentication is successful, the POP client goes to the Transaction State, in this state LIST, RETR and DELE commands can be used to display, download and delete messages from the server, respectively. Messages defined for deletion are not actually removed from the server until the POP client sends the QUIT command to terminate the session. At this point, the POP server goes into the Update State, a phase in which the flagged messages are deleted and all the remaining resources of the session are cleaned up.

You can manually connect to the POP3 server by Telnet to port 110. It is very useful when they send you a message with a very long file that you do not want to receive.

Some orders

  • User (name): User identification (Only done once).
  • Pass (password): You send the server key.
  • Stat: Gives the number of messages not deleted in the mailbox and their total length.
  • List: Shows all messages not deleted with their length.
  • Retr (number): Requests the sending of the message specifying the number (it is not deleted from the mailbox).
  • Top (number) (lines): Displays the header and the required number of lines of the message specifying the number.
  • Dele (number): Delete the message by specifying the number.
  • Rset: Recover deleted messages (on current connection).
  • Quit: Exit.

Post Office Protocol