What Is POP3? (Post Office Protocol Version 3 Explained)

POP3 stands for Post Office Protocol version 3. It’s an email retrieval protocol that works by downloading your messages from a mail server to your device. Once downloaded, messages are typically removed from the server. They live on your computer or phone, not on the server, and other devices can’t access them.

That distinction matters more than it might sound. It’s the reason most people shouldn’t be using POP3 today, and the reason it made perfect sense when it was designed in the early 1980s, before most people had more than one device that could receive email.

How POP3 Works

When your email client connects using POP3, it follows a straightforward sequence. It connects to the mail server, authenticates with your username and password, retrieves a numbered list of messages waiting in your inbox, and downloads them to your local device one by one. By default, it then marks them for deletion on the server. When the session closes, the server removes those messages. The next time another device connects, there’s nothing left to retrieve.

The protocol is deliberately minimal by design. It has no concept of folders beyond the inbox. It can’t synchronise read status, flags, or folder organisation across devices. It can’t report which messages you’ve already seen on another device. It was built to do one thing efficiently: let a machine pick up its mail and disconnect. In an era of expensive dial-up connections charged by the minute, that simplicity was a feature.

Quick Reference

POP3 downloads your email from the server to your device and removes it from the server by default. It runs on port 110 unencrypted or port 995 over TLS. Always use 995. The current specification is RFC 1939, published in 1996. It doesn’t support multi-device access or folder synchronisation, which is why most people should use IMAP instead.

POP3 Ports

POP3 uses two ports:

• Port 110 is the standard POP3 port, unencrypted. Your username and password are sent in plaintext, which is a significant security risk on any network you don’t fully control, including most public Wi-Fi.
• Port 995 is POP3 over TLS, sometimes called POP3S. The connection is encrypted from the start. This is the only port you should use for any modern email setup.

If you’re configuring an email client manually and your host offers both options, always choose port 995. Port 110 without encryption has no place on a modern network. RFC 8314, published in 2018 and titled “Cleartext Considered Obsolete,” formally deprecated unencrypted email protocols for new implementations. That applies to POP3 on port 110 just as it does to IMAP on port 143.

The Leave a Copy Option

Most email clients offer a setting called “leave a copy of messages on the server” when configured with POP3. This prevents automatic deletion after download, so a second device connecting later can also retrieve the messages.

It sounds like a solution to the multi-device problem, but it’s a workaround with real limitations. There’s still no synchronisation. If you read a message on your phone, your laptop will still download it and show it as unread. If you delete a message on your laptop, it stays on your phone. If you move a message to a folder on your desktop client, that folder only exists locally. The server still has everything sitting in the inbox. Two devices using POP3 with “leave a copy” enabled don’t share a mailbox. They each have their own independent copy of whatever the server held at the moment they connected.

If you find yourself relying on this setting, IMAP is almost certainly the right choice. It solves the multi-device problem properly rather than working around it.

POP3 vs IMAP

The fundamental difference is where your email lives. POP3 downloads messages to your device and removes them from the server. IMAP keeps messages on the server and synchronises every action: reads, deletions, folder moves, flags, all of it, across all your devices in real time.

For anyone who accesses email from more than one device, IMAP is the better choice. POP3 creates fragmented inboxes with no way to reconcile them. A message downloaded to your phone before your laptop connects is gone from the server before your laptop ever sees it.

That said, POP3 does have two genuine advantages worth acknowledging. First, it works fully offline. Once messages are downloaded, you can read, reply to, and organise them without an internet connection, and replies queue until you reconnect. Second, it uses less server storage, since messages are removed after download. For hosting plans with very tight mailbox limits, this can occasionally be relevant.

Neither advantage outweighs the synchronisation problem for most users. The offline argument has weakened as always-on mobile data has become standard. The storage argument is rarely decisive given how cheaply storage is provided on modern hosting plans. POP3 remains in use primarily for backward compatibility and niche cases, not because it’s the better protocol for the average person.

POP3 and Email Migration

If your email account was configured using POP3, migrating when switching hosting providers works very differently to an IMAP migration. With IMAP, your messages are on the server and can be copied across remotely. With POP3, messages were downloaded to your local device and removed from the server. There’s nothing left on the server to migrate.

In practice this means a POP3 migration doesn’t involve copying messages between servers at all. You reconnect your email client to the new server after switching. Your existing downloaded messages stay on your device where they are. New messages start arriving at the new server and your client downloads them as normal. The transition is simpler than an IMAP migration in technical terms, but it only works cleanly if you’re on a single device. If you’ve been using “leave a copy” across multiple devices, you need to work out which device has the complete picture before you switch.

Our guide to migrating email when switching hosting providers covers both POP3 and IMAP scenarios, including the DNS changes needed to route new mail to the right server during the transition.

A Brief History

The original Post Office Protocol was published in October 1984 by Joyce K. Reynolds at USC’s Information Sciences Institute as RFC 918. The problem it solved was concrete and pressing: personal computers of the era couldn’t stay connected to the internet. They had dial-up modems, limited memory, and no always-on networking. Email existed on servers that stayed connected. Reynolds’ protocol gave small machines a way to visit the server, collect waiting mail, and disconnect, like a post box rather than a permanent address.

The protocol was five pages long and deliberately simple. POP2 followed in 1985 with RFC 937, adding modest improvements. POP3 was specified by Marshall Rose in RFC 1081 in November 1988 and became the dominant version. The current specification is RFC 1939, published in May 1996, which remains the definitive standard today.

Reynolds went on to co-lead the RFC Editor function alongside Bob Braden for many years and received the Postel Award in 2006 for her contributions to the internet. She died in December 2015.

As internet connections became always-on and people started checking email on multiple devices, the limitations of POP3 became more apparent. IMAP, designed to address exactly those limitations, gradually became the standard. Most modern email clients still support POP3 for backward compatibility, but IMAP is the default for any new account setup.