Updated 8 hours ago
Before IMAP, email had a problem: your messages could only really live in one place. Download them to your laptop, and your phone couldn't see them. Read something on your phone, and your laptop wouldn't know. Every device had its own version of your mailbox, and keeping them in sync was your problem.
IMAP flipped this around. Instead of downloading messages to your devices, IMAP keeps them on the server. Your devices don't hold copies of your mailbox—they're windows into it. Read an email on your phone, and it's marked as read everywhere. Delete something on your laptop, and it vanishes from your tablet. The server is the truth; your devices just show you that truth.
The Conversation Between Client and Server
IMAP is a dialogue. Your email client talks to the server, asking questions and giving instructions. The server responds with information and confirmations.
Connecting and Logging In
The client connects (typically port 993 for encrypted connections) and authenticates:
Every client command starts with a tag (A001, A002, etc.) so responses can be matched to requests.
Opening a Mailbox
To work with messages, the client selects a folder:
The server immediately tells you what's inside: 172 messages total, 5 arrived recently, message 12 is the first you haven't read. This is why your email client can show you an unread count before downloading anything.
Fetching Messages
The client can ask for exactly what it needs:
This retrieves just the flags and envelope (sender, subject, date) for messages 1 through 5—enough to display a message list without downloading full content.
Two Ways to Point at a Message
IMAP has two numbering systems for messages, and understanding why reveals something about the problem it's solving.
Sequence numbers are simple: message 1 is the first in the folder, message 2 is the second, and so on. But delete message 5, and what was message 6 becomes message 5. Every message after it shifts down. Sequence numbers are convenient for "give me the first 10 messages" but terrible for "remember this specific message."
UIDs (Unique Identifiers) never change. A message assigned UID 47293 keeps that UID forever, even if thousands of other messages are deleted around it. UIDs only go up—newer messages always have higher UIDs than older ones.
Clients use sequence numbers for quick operations within a session. They use UIDs to track specific messages across sessions. Your email client remembers "I was looking at UID 47293," not "I was looking at message 5."
Flags: How State Syncs Across Devices
Flags are how IMAP tracks what you've done with messages:
- \Seen — You've read it
- \Answered — You've replied
- \Flagged — You've starred or flagged it
- \Deleted — Marked for deletion
- \Draft — It's a work in progress
When you star an email on your phone, your client tells the server:
Now that flag exists on the server. When your laptop checks in, it sees the flag and shows you the star. The server is the truth; your devices reflect it.
The Deletion Dance
Deleting in IMAP is a two-step process. First you mark messages as deleted:
But they're still there—just marked. To actually remove them, you expunge:
This is where the "Trash" metaphor in email clients comes from. Moving to trash marks as deleted. Emptying trash expunges.
Fetching Only What You Need
IMAP doesn't force you to download entire messages. This matters when you're on a slow connection or when messages have large attachments.
Headers only:
Enough to display who sent it and the subject line.
Just the text part:
For a multipart message, grab just the text without the 50MB attachment.
A preview:
The first 1000 bytes—enough to show a snippet.
This selective fetching is why your email app can show you a list of hundreds of messages almost instantly, then load full content only when you tap on one.
Searching Without Downloading
IMAP can search on the server:
The server does the work and returns just the matching message numbers. Your client didn't have to download every message to find these three. For a mailbox with 50,000 messages, this is the difference between a one-second search and a 20-minute download.
Real-Time Updates with IDLE
Without IDLE, email clients poll: check for new messages every few minutes. This wastes battery, wastes bandwidth, and means delays before you see new mail.
IMAP IDLE lets the client say "I'll wait here—tell me when something changes":
The server pushes a notification the moment a new message arrives. The client can then fetch it immediately. This is how your phone buzzes within seconds of receiving an email.
Folders and Hierarchy
IMAP organizes messages into folders (the protocol calls them mailboxes, but everyone calls them folders). INBOX is standard; everything else is up to you:
Create a folder on your laptop, and it appears on your phone. The folder structure lives on the server, just like the messages.
Security
IMAP connections should always be encrypted. Two approaches:
Implicit TLS (port 993): The connection is encrypted from the first byte, like HTTPS.
STARTTLS (port 143): Connect unencrypted, then upgrade:
Modern servers refuse to accept passwords until encryption is active.
Beyond passwords, IMAP supports OAuth2 (used by Gmail and Office 365), where your email client gets a token instead of storing your password directly.
Synchronization and Offline
Modern email clients cache messages locally. This lets you:
- Read messages without a connection
- Queue actions (delete, flag, move) while offline
- Sync changes when connectivity returns
The synchronization challenge: what if you flag a message on your phone while offline, and delete it on your laptop? Clients handle this differently—some use last-write-wins, some prompt you, some try to merge. The protocol doesn't dictate a solution; it just provides the building blocks.
Extensions That Matter
IMAP's core is decades old, but extensions keep it current:
- SORT — Server sorts messages so your client doesn't have to
- THREAD — Server groups conversations
- QUOTA — Query how much storage you've used
- QRESYNC — Efficiently sync only what's changed since last time
- COMPRESS — Compress the connection, saving bandwidth
Clients discover what's available:
Common Frustrations
Large mailboxes slow down. With hundreds of thousands of messages, operations take time. Archiving old mail or splitting into subfolders helps.
Connection limits. Servers cap concurrent connections per account. Multiple devices plus multiple apps can hit this ceiling.
Storage quotas. Since messages stay on the server, you can run out of space. Your client might tell you; your server administrator definitely will.
Folder delimiter confusion. Some servers use / between folder levels, some use .. This occasionally causes folder sync issues between different clients.
Frequently Asked Questions About IMAP
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