What Is IMAP? (Internet Message Access Protocol Explained)

IMAP stands for Internet Message Access Protocol. It’s the protocol your email client uses to connect to a mail server and access your messages. What makes it different from older approaches is where your email actually lives: with IMAP, messages stay on the server. Your phone, laptop, and tablet all connect to the same mailbox and see the same thing.

That sounds obvious now. In 1986, when Mark Crispin designed the original protocol at Stanford University, it wasn’t obvious at all. At the time, most people retrieved email by downloading it to a single computer and removing it from the server. IMAP was built around the opposite idea: leave everything on the server and let any device access it, at any time, without disrupting what other devices see.

How IMAP Works

When you open your email client and your inbox loads, IMAP is what makes that happen. The client connects to the mail server, authenticates with your username and password, and retrieves the list of messages in your mailbox. The messages themselves stay on the server. Your client downloads headers first: sender, subject, date. It only pulls the full message body when you open it. This is more efficient than downloading everything at once, especially on mobile connections.

Every action you take is recorded on the server, not just on your device. Read a message on your phone and it appears as read on your laptop the next time you check. Move an email to a folder and that folder structure is mirrored everywhere. Delete a message and it disappears across all your devices. Flag an email for follow-up and the flag is there when you open your desktop client later. The server is the single source of truth, and every client reflects it.

This architecture also means your email survives device loss or failure. If your phone is stolen or your laptop dies, every message is still sitting on the server. Log into a new device, configure your email account, and your full inbox history is there exactly as you left it.

Quick Reference

IMAP stores your email on the server and synchronises it across every device. It runs on port 143 unencrypted or port 993 over TLS. Always use 993. The current specification is RFC 9051, published in 2021. It supports multiple devices and full folder synchronisation, making it the standard choice for virtually all modern email setups.

IMAP Ports

IMAP uses two ports:

• Port 143 is the standard IMAP port. It supports optional encryption via STARTTLS, where the connection starts unencrypted and can be upgraded to TLS mid-session.
• Port 993 is IMAP over TLS, sometimes called IMAPS. The connection is encrypted from the first byte, with no plaintext phase at all.

RFC 8314, published in January 2018, settled a long-running debate about which approach was better. Its title is unambiguous: “Cleartext Considered Obsolete.” The RFC recommends implicit TLS on port 993 for all new implementations. The reasoning is straightforward. STARTTLS on port 143 introduces a negotiation window where an attacker could strip the upgrade command before it reaches the server, downgrading your connection to plaintext without you knowing. Port 993 has no such window. If you’re configuring an email client and see port 143 without a TLS option, switch to 993.

What IMAP Stores on the Server

IMAP keeps the following on the server:

• All messages in every folder
• Read and unread status
• Replied, forwarded, and flagged status
• Folder structure and organisation
• Draft messages (when supported by the client)

What it doesn’t carry over automatically when you switch hosting providers: filters, forwarders, autoresponders, contacts, calendars, and email signatures. Those live in your host’s mail server configuration or your local email client, not in the IMAP message store. You have to recreate them manually on the new server. Our guide to migrating email when switching hosting providers covers exactly what moves and what doesn’t, including the DNS timing you need to get right to avoid losing mail in transit.

IMAP vs POP3

The key difference is where your messages live. IMAP keeps them on the server. POP3 downloads them to your device and typically removes them from the server after download.

For most people, IMAP is the right choice. If you check email on a phone and a laptop, which is nearly everyone, POP3 creates an immediate problem: messages downloaded to your phone disappear from the server, so your laptop never sees them. You end up with two separate, incomplete inboxes on two devices.

POP3 does have one genuine advantage: it works completely offline once the download is done. In an era of expensive dial-up connections and unreliable internet, this mattered. For most users today, with always-on mobile data, it rarely does.

IMAP requires more server storage, since messages accumulate on the server rather than being cleared after download. It’s worth checking before you commit to a hosting plan. Some providers cap individual mailboxes at 1 GB or 2 GB, which fills up faster than you might expect if you receive attachments regularly.

IMAP and Your Hosting Plan

Almost every web hosting plan that includes email supports IMAP by default. When you create a custom email address through your host’s control panel and configure it in Outlook, Apple Mail, or Thunderbird, you’ll be given IMAP server settings. These are typically an incoming server address like mail.yourdomain.com on port 993, and an SMTP outgoing server address on port 465 or 587 for sending.

IMAP support is standard across all serious hosting providers and not a differentiator. What does vary is mailbox storage. Some shared hosting plans offer unlimited mailbox storage; others impose per-account limits of 1 GB, 5 GB, or a total allocation across all accounts on the plan. If you’re running a business with multiple team email addresses, check this before signing up rather than discovering the limit when inboxes start bouncing.

Some hosts offer dedicated email hosting separately from web hosting, such as Google Workspace or Zoho Mail. These run on their own infrastructure and you point your domain’s DNS MX records at them rather than at your web host. The trade-off is additional cost for better reliability, storage, and features. Most small sites are well served by the email hosting included with their web hosting plan.

After setting up or migrating email, you can use the DNS Lookup tool to verify your MX records are pointing to the correct mail server. Enter your domain and select MX from the record type menu.

A Brief History

Mark Crispin created the original IMAP protocol at Stanford University in 1986, initially calling it the Interim Mail Access Protocol. He was watching POP being widely deployed and felt its design was too limited. It offered no way to manage a mailbox remotely, only to retrieve its contents. His alternative let users treat a remote mailbox as if it were local: create folders, move messages, mark mail as read, all without downloading anything permanently.

The protocol went through several iterations. IMAP2, defined in RFC 1064 in 1988, introduced command tagging, which let clients send multiple commands without waiting for each response. IMAP3 appeared briefly in 1991 and was rejected by the market. The IETF working group returned to IMAP2 and evolved it into IMAP4, adding MIME support and more complete mailbox management. RFC 3501, published in March 2003 and authored by Crispin, defined IMAP4rev1. That version became the foundation of modern email for nearly two decades. Crispin also co-created Pine, the email client that introduced many people to Unix email. He died in December 2012 at the age of 56.

The current specification, RFC 9051, was published in August 2021, defining IMAP4rev2 with updated security requirements and improved handling of international characters in email addresses and folder names.