What is POP3? The download-and-delete email protocol

What POP3 is

POP3 stands for Post Office Protocol version 3. It is the older of the two common ways for a mail app to collect mail. The model is a post office box: you go to the box, take everything out, and carry it home. Once the mail is on your computer, the server's job is done.

The current version is defined in RFC 1939, published in 1996. It is still supported by most providers, but it has not grown much since, and nearly all new setups use IMAP instead.

POP3 made sense when it was designed. People connected by dial-up and paid by the minute, and servers had little disk space. Downloading everything in one short session and reading it offline was the efficient choice.

How a POP3 session works

POP3 is very small. A client signs in, asks how many messages there are, downloads each one, optionally marks them for deletion, and disconnects. Deletions happen when the client sends QUIT. A full session can look like this:

S: +OK POP3 server ready
C: USER alice@example.com
S: +OK
C: PASS ********
S: +OK Logged in
C: STAT
S: +OK 2 4820
C: RETR 1
S: +OK 2210 octets
S: (message text)
S: .
C: DELE 1
S: +OK Message 1 deleted
C: QUIT
S: +OK Bye

Leaving mail on the server

Most mail apps offer a setting like leave a copy of messages on the server. With it on, the client downloads mail but does not send DELE. To avoid downloading the same message twice, it uses the UIDL command, which gives each message a stable ID the client can remember.

This lets two computers both download the same mail, but each one keeps its own separate state. Reading a message on one does not mark it read on the other, and folders you create stay on the computer where you made them.

Ports and limits

POP3 uses port 110 for plain connections (which may upgrade with STARTTLS) and port 995 for connections encrypted with TLS from the start. Use 995 whenever your provider offers it.

POP3 only covers the inbox. It has no folders, no server-side search, no flags and no way to push new mail to you, so clients check on a timer. Like IMAP, it cannot send mail. That is still SMTP's job.

Because the server keeps no record of what you have read or sorted, every POP3 device starts from scratch. Sent messages are saved only on the computer that sent them, unless you also BCC yourself or the provider saves them for you.