Port 143: IMAP (Email Retrieval)

Before IMAP, your inbox was a place. It lived on one computer, and if you wanted to read your email, you went to that computer. Download a message on your work desktop, and it vanished from everywhere else. Your laptop couldn't see it. Your phone didn't exist yet, but even if it did, it wouldn't matter.

This was POP3—the Post Office Protocol. Your mail server was a post office box: you showed up, grabbed your mail, and took it home. Once downloaded, the messages disappeared from the server. Fine for one computer. Useless the moment you wanted two.

IMAP took the opposite approach. Your messages stay on the server. Always. Your email client doesn't download mail—it views it. Think of it as the difference between downloading a movie versus streaming from Netflix. The movie exists in one place, but you can watch it from any screen.

This transformed email from a physical location into an idea. Read an email on your phone, and it's marked as read on your laptop. Create a folder on your tablet, and it appears everywhere. Delete something via webmail, and it's gone from all your devices.

Port 143 is where this happens.

How IMAP Works

When your email client connects to port 143, it opens a conversation with the mail server. This isn't a quick handshake—it's a persistent connection that can stay open for hours.

First, your client authenticates. It sends your username and password (or an OAuth token) to prove you're allowed in.

Then it asks what's available. "What folders exist? How many messages in the inbox? What's new since I last checked?" The server responds with the current state of your mailbox.

From there, your client can do anything: fetch message headers without downloading full bodies, move messages between folders, search for emails matching specific criteria, mark things as read or starred. Every action happens on the server. Your client is just a window into mail that lives somewhere else.

The IDLE Command: Why Your Phone Buzzes Instantly

Without IDLE, your email client would constantly ask the server "Any new mail? How about now? Now?" This polling wastes bandwidth, drains batteries, and still introduces delays.

IDLE flips the relationship. Your client tells the server: "I'm listening. Let me know when something happens." Then it waits. The connection stays open, silent, consuming almost nothing.

The moment a new email arrives, the server immediately tells your waiting client. No delay. No wasted polling.

This is push email. It's not magic—it's a client that knows how to wait and a server that knows how to speak up.

Folders That Follow You

IMAP doesn't just sync messages—it syncs your entire folder structure. Create "Work/Projects/2024" on your laptop, and it appears on your phone. Move a message into it from your tablet, and every device sees the move.

The protocol treats folders as a hierarchy, like a file system. Servers nest them using delimiters (usually "/" or "."), creating arbitrarily deep structures that replicate perfectly across every client.

IMAP also defines special-use folders. Instead of clients guessing which folder is Trash or Sent, the server announces: "This one is for drafts. This one is for spam." Your phone knows where to save drafts even if the folder is called "Brouillons" on a French server.

The Problem with Port 143

Everything sent through port 143 travels in plaintext. Your password, your messages, your entire email history—readable by anyone watching the network.

On coffee shop WiFi, an attacker could capture your credentials and access your account later. They could read every message you read. They could modify messages in transit before they reach you.

This isn't theoretical. Port 143 should never touch an untrusted network.

Port 993: The Encrypted Version

Port 993 runs IMAPS—IMAP wrapped in TLS encryption from the first byte. No negotiation, no "start unencrypted and upgrade later." Encrypted immediately, stays encrypted.

Your credentials are protected. Message content is unreadable to observers. The TLS certificate verifies you're talking to the real server, not an imposter.

Modern email services use port 993 by default. Many have disabled port 143 entirely. If your email client is configured correctly, it's already using encryption—you'd have to go out of your way to expose yourself.

When Port 143 Still Exists

Some organizations keep port 143 for internal networks where the network itself is trusted and encryption overhead matters. Legacy systems that can't handle TLS might require it. Testing sometimes benefits from seeing unencrypted traffic.

Edge cases. For normal email over the Internet, port 993 is the only responsible choice.

What IMAP Changed

IMAP moved our inboxes from hard drives to servers before "cloud" was a word anyone used. It made email something you could access from anywhere, on any device, and find exactly where you left off.

Port 143 carries that capability. Port 993 carries it safely. Same protocol, same power—one exposes everything you send, the other protects it.

Use the protected one.