IMAP
Internet Message Access Protocol — the standard protocol for accessing email stored on a server, keeping messages synchronized across multiple devices without downloading and deleting them.
IMAP (Internet Message Access Protocol, RFC 9051) is the protocol that modern email clients use to connect to mail servers such as Gmail, Outlook.com, and corporate Exchange servers. Unlike its predecessor POP3, IMAP keeps messages on the server and synchronizes the client's view of the mailbox, so the same messages appear consistently whether you read them on a phone, a laptop, or the web.
Gmail exposes its mailbox structure over IMAP, mapping labels to IMAP folders. This means any standard IMAP client can access Gmail, though Gmail-specific features such as threaded conversations are not part of the IMAP standard. When you want a fully offline, searchable copy of your IMAP mailbox, the most common approach is to use a client like Thunderbird to download everything and then export to MBOX.
IMAP is a live connection protocol, not an archival format. For long-term preservation of email, exporting to MBOX or EML is recommended, because those formats do not depend on a server being available. Google Takeout automates this export step for Gmail users.
Related terms
Post Office Protocol 3 — an older email retrieval protocol that downloads messages from a server to a local device, typically removing them from the server afterward.
Simple Mail Transfer Protocol — the standard protocol used to send and relay email messages between mail servers. It is used for outgoing mail only; reading email requires IMAP or POP3.
A mailbox format that stores each email message as a separate file within a directory hierarchy, rather than concatenating all messages into a single file like MBOX.