Based on the TCP/IP protocol suite, Post Office Protocol (POP) is the language that desktop email clients or webmail services such as Gmail, Yahoo, Outlook, etc. use to interact with email servers to retrieve incoming emails and store them locally, on a computer. This ensures the mails are accessible even when there is no Internet.

POP is a protocol that facilitates a part of email communication. It works in tandem with another protocol called Simple Mail Transfer Protocol (SMTP) to get the job done. SMTP is a protocol used by mail clients to send mails to an email server. POP also helps save server storage space since the mail gets deleted from the server after download. There is also the option to keep the mail on the server after download to facilitate future downloads or access from other Internet-enabled devices.

Email Message Retrieval

The POP server retrieves the message from the SMTP server. The email client establishes a TCP session using port 110 to connect to the POP server, so that it can make itself known to the server and communicate with it. Once the session has been created, the email client asks the server the total messages waiting for retrieval and each message’s size. Then, individual messages are retrieved and the POP session is terminated.

POP Versions

POP has been updated twice since its origin. The first version, POP, was published in 1984. POP2 and POP3 happened in 1985 and 1988, respectively.

New versions are published to expand the capabilities of the protocol or encompass new features. For instance, POP3 enabled downloading email messages on a local computer and reading them offline. POP3 can also receive and send an email message using only POP, and is also capable of using SMTP to send emails. POP2, on the other hand, needed SMTP to deliver messages.

POP3 version has been defined in RFC (Request for Comments) 1725. RFC is formal documentation defining evolving Internet standards. POP and POP2 were initially defined in RFC 918 and RFC 937, respectively.

POP Ports

A port helps a POP server connect to and interact with the user’s email client. The default POP port is port 110. But it’s non-encrypted and not secure as port 995, which is also called POP3S. Port 465 is an alternative POP port as well but it’s not secured.

Comparing POP and IMAP

As aforementioned, the majority of email clients use POP to download email messages from a mail server to store them locally. But not all email applications use POP. In fact, quite a few have switched to Internet Message Access Protocol (IMAP) and others are contemplating the move. This is primarily because IMAP can synchronize a user’s mailbox across different devices, which POP cannot. Also, IMAP is a lot more complex in core design and is therefore significantly more advanced and capable than straightforward POP.