What Is IMAP? A Plain-English Definition

IMAP stands for Internet Message Access Protocol. It's the standard, defined in RFC 3501, that lets an email client (Outlook, Apple Mail, Thunderbird, your phone) read mail stored on an email server.

The key idea behind IMAP: your mail stays on the server. Clients view it, manipulate it, and synchronise state — but the master copy lives server-side. This is what makes the same inbox look identical on your laptop, your phone, and the web client.

How IMAP differs from POP3#

The older protocol, POP3, was designed for a world where you connected briefly, downloaded your mail, and disconnected. POP3 pulls mail to your client and (by default) deletes it from the server. That worked in 1995. It does not work in a world where you read mail on five devices.

Aspect	IMAP	POP3
Where mail lives	On the server	On your client
Multiple devices	Sync identically	Each device gets a separate copy
Folder support	Yes, including nested	No (just inbox)
Server-side search	Yes	No
Best suited to	Modern multi-device users	Single-device, offline-heavy

For email migrations, IMAP is overwhelmingly the protocol of interest. Almost every modern provider speaks IMAP, and a migration tool can connect to both ends, read messages from one, and write them to the other.

What IMAP can and cannot do#

IMAP moves mail. It does not move:

• Calendar events
• Contacts
• Tasks
• Out-of-office and forwarding rules
• Custom signatures

To move those, you need additional protocols (CalDAV, CardDAV) or vendor-specific tools (EWS, Graph API). This is why "I migrated my IMAP mailbox" rarely covers everything a user actually needs migrated.

Why IMAP matters for migrations#

Because IMAP is universal, it's the path of least resistance for moving mail between providers. Mailbox Taxi connects to IMAP on both ends, reads messages in batches, preserves folder structure and message metadata (date, flags, read/unread), and writes them to the destination.

The trade-off is that IMAP is slower than vendor-native protocols. A Microsoft EWS-based migration between two Exchange servers will be 2–3× faster than the same move via IMAP. But for cross-provider migrations (Gmail → Outlook, IMAP → IMAP, Yahoo → Workspace), IMAP is the only option that works everywhere.

For the broader context, see the complete email migration guide, or jump straight to the Gmail to Outlook walkthrough.