What is POP3, IMAP and SMTP?

3 minute read

Emails make the world go round… Well, not quite, but it’d be hard to imagine the world without them. Nowadays, the vast majority of communication is done via emails, whether it’s a receipt for an online shop, sending some holiday pictures to a friend or something else – emails are an fast and easy way of sending them.

As with everything related to technology, there are underlying protocols and systems which make it function. You’ve probably heard of these ones: POP3, IMAP and SMTP.


POP3 stands for Post Office Protocol 3 and is used for email retrieval by a client from a remote server. Periodically the email client (Outlook, Thunderbird etc) will check the server for new emails and if there are any, download them. By default POP3 will remove each email from the server once it’s downloaded it. Think of it literally, “Post Office Protocol” - the Post Office receives a message and then deliver it, they don’t make a copy, it’s the same with POP3. Some administrators have configured their servers to keep a copy however.

POP3 has a couple of disadvantages, mainly, you can configure your account in multiple email clients but the first one to download the message is the only one that will get it. Also, as a single client is the only device that has a copy there’s no backup. If anything happens to it you’ve lost all your emails for good.


IMAP is much like POP3, it’s also a email retrieval protocol used by the client to fetch emails from the server. It stands for Internet Message Access Protocol. The main difference is that the server keeps the primary copy and the client(s) cache a local copy and will upload any additions/changes to the server immediately. This means you can have as many devices connected to your email account as you like and they will all be up to date and in sync. Send an email from one and it appears in the “Sent Items” on the others. IMAP is the protocol you should be using wherever possible.

IMAP also only downloads the email headers until the client explicitly requests more - this means that you get a faster overview.


SMTP stands for Simple Message Transfer Protocol or, if you want an easy way of remembering what it does “Send Mail To People”. This perfectly describes what SMTP does, it’s the protocol for transferring emails. When you send an email your mail client sends it to your mail server using SMTP, your mail server then sends it to the receiving person’s mail server using SMTP then they download it with POP3 or IMAP.


When you configure an email client one of the entries is which ports to use. Here’s a list of the standard ones.

  • IMAP - Encrypted (SSL or TLS): 993, Unencrypted: 143
  • POP3 - Encrypted (SSL or TLS): 995, Unencrypted: 110
  • SMTP - Encrypted (SSL or TLS): 465 or 587, Unencrypted: 25

Shortlink: on-te.ch/sip