POP3 (Post Office Protocol)

POP3 (Post Office Protocol version 3) is an email retrieval protocol that downloads messages from a mail server to your local device. First standardized in 1988, POP3 represents a fundamentally different philosophy of email than what we use today.

Here's the core insight: POP3 treats your mailbox like a physical mailbox. You walk to the post office, take your letters out, bring them home. The post office no longer has them—you do. This made perfect sense when people had one computer, storage was expensive, and you needed to read email offline.

IMAP, by contrast, treats your mailbox like a library. The books stay on the shelves. You visit them, read them, maybe bookmark pages—but they remain at the library for your next visit, or for anyone else who needs them.

This difference explains everything about why POP3 is fading and IMAP dominates.

How POP3 Works

The protocol is beautifully simple. Your email client connects to the server, logs in, downloads waiting messages, and optionally deletes them from the server. Done.

A typical session:

S: +OK POP3 server ready
C: USER alice
S: +OK
C: PASS secret
S: +OK Logged in
C: STAT
S: +OK 3 4500
C: RETR 1
S: +OK 1500 octets
S: [message content]
S: .
C: DELE 1
S: +OK message 1 deleted
C: QUIT
S: +OK Bye

STAT tells you there are 3 messages totaling 4,500 bytes. RETR downloads a message. DELE marks it for deletion. QUIT ends the session—and only then are deletions actually applied. If your connection drops before QUIT, nothing is lost.

The entire command set fits on an index card: USER, PASS, STAT, LIST, RETR, DELE, QUIT, plus a few optional commands like TOP (peek at headers) and UIDL (get unique message identifiers). This simplicity made POP3 trivial to implement and utterly reliable.

The "Leave on Server" Workaround

When people started wanting to check email from multiple computers, POP3 clients added a "leave messages on server" option. Instead of deleting after download, messages stay put.

This seems to solve multi-device access. It doesn't.

Here's what actually happens: each device downloads its own copy of every message. You now have independent collections that happen to start from the same source. Delete a message on your laptop? It's still on your phone—you deleted your copy, not the original. File something into a folder? That organization exists only on that device. Mark something as read? Other devices don't know.

It's like photocopying your mail before taking it home. Sure, everyone can have a copy now. But if you write notes on your copy, annotate the margins, throw some letters away—none of that affects anyone else's copies.

The problem isn't keeping copies. The problem is that everyone needs to see the same mailbox. That's what IMAP was designed for.

POP3's Limitations

POP3's simplicity is also its constraint:

No folders. POP3 has no concept of organization. Everything comes from one inbox. Any filing you do is local to that device.

No flags. There's no read/unread, no stars, no categories. Messages exist or they don't.

No search. You can't ask the server to find messages matching criteria. Download everything first, then search locally.

No selective fetch. Want just the subject lines to decide what's worth downloading on a slow connection? Too bad. RETR gives you the whole message or nothing.

No synchronization. This is the fundamental issue. Each device maintains its own independent reality.

When POP3 Still Makes Sense

Despite everything, POP3 has legitimate uses:

Archival. Want to pull all messages off a server into local storage? POP3 does exactly this. Download, delete from server, store locally forever.

Single device, offline access. If you genuinely access email from one computer and need everything available without Internet, POP3's download-and-store model is actually ideal.

Server storage limits. Some cheap or legacy systems have tiny mailbox quotas. POP3's download-and-delete keeps the server clear.

Backup. Periodic POP3 retrieval to a local archive, separate from your main IMAP access, creates a backup copy you control.

Security

Original POP3 sent everything—including passwords—in plain text. Modern implementations require encryption:

Port 995 (POP3S): Encrypted from the start, like HTTPS.

Port 110 with STARTTLS: Starts unencrypted, upgrades to TLS before authentication.

Never connect to a POP3 server without encryption. If a server doesn't support TLS, don't use it.

The Protocol's Decline

POP3 was dominant through the 2000s. Then smartphones happened.

Suddenly everyone accessed email from multiple devices. The download-and-delete model made no sense when you wanted to start reading on your phone and finish on your laptop. IMAP's synchronized, server-centric approach became essential.

Today, many providers disable POP3 by default or don't support it at all. Gmail requires you to explicitly enable it. The protocol persists mainly for legacy compatibility and specific archival workflows.

The design that made POP3 perfect for 1988—when you had one computer, dialup Internet, and expensive storage—is exactly what makes it wrong for 2025. The protocol didn't fail. The world changed around it.