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IMAP (Internet Message Access Protocol) lets an email app access and manage messages and folders kept on a mail server. Because the server holds the mailbox, a phone and a computer can each update their view of the same mailbox when they connect. IMAP does not send newly composed email; a separate mail-submission protocol handles that.
What does IMAP mean?
IMAP is a client-server protocol for accessing and manipulating email messages in server-side mailboxes. The IETF’s IMAP4rev2 standard describes it this way: “The Internet Message Access Protocol Version 4rev2 (IMAP4rev2) allows a client to access and manipulate electronic mail messages on a server.” (RFC 9051, August 2021.)
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In practical terms, your email app is the client, and your provider’s system is the server. The client can select a mailbox, search messages, fetch information or message content, and update message flags. The exact features available depend on the server and client.
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How does IMAP keep email in sync?
Synchronization is not necessarily a constant, complete copy of every message to every device. It is a series of exchanges in which a client checks server-held mailbox information and updates its own view. A client may store local data for display or offline access, but its caching and refresh behavior are implementation choices.
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- Connect and select a mailbox. The client connects to the server, authenticates, and selects a mailbox such as an inbox. IMAP commands and server responses govern the exchange.
- Check mailbox information. The client can ask about messages and search or fetch selected attributes. It can retrieve message content when needed; it does not have to download every message body just to learn about mailbox state.
- Keep a local view. Depending on the app, it may cache message details, content, or flags so the mailbox can be displayed and some work can continue offline. The degree of offline access varies by app.
- Reconcile after reconnecting. When the client reconnects, it compares its cached information with the server and fetches or refreshes what is needed. Guidance for disconnected clients is described in RFC 4549.
- Apply and report changes. The client can update flags, while the server can report observed changes such as mailbox size or message flags. How quickly you see those updates depends on the client, provider, and connection behavior.
What do UIDs and flags do?
UIDs identify messages across sessions
A message sequence number is its current position in a selected mailbox, so it can change when mailbox contents change. A UID is a server-assigned identifier intended to remain stable within that mailbox’s UID-validity context, allowing a client to refer to messages when it resynchronizes.
Each mailbox also has a UIDVALIDITY value. If that value changes, the client must treat its old UID mapping for that mailbox as invalid and rebuild or reconcile its cache rather than assuming old identifiers still apply. UIDNEXT is the server’s predicted UID for a new message; it can help a client notice possible arrivals, but does not guarantee that a message will receive that exact value. These rules are set out in RFC 9051 and synchronization guidance in RFC 4549.
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Flags carry common message state
IMAP flags can record states such as \Seen (seen), \Answered, \Flagged, \Deleted, and \Draft. A server may permit some flags to persist and others to be session-only, so behavior can vary. In particular, \Deleted marks a message for later removal; it is not itself the same as immediately expunging the message permanently.
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No. IMAP accesses and manages messages already in server mailboxes; it does not define how a newly composed message is submitted for delivery. The email app may use a separate mail-submission protocol, such as the one described in RFC 6409. Because an app can handle mailbox access and message submission behind one interface, they may appear to be a single function even though they use distinct protocols.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Are all IMAP features the same across providers?
No. IMAP defines common protocol behavior, but providers can add extensions and clients can implement features differently. Google documents Gmail-specific extensions that clients can discover through the IMAP CAPABILITY mechanism. Among them are Gmail label-related behavior, special-use folder attributes, and attributes for Gmail message and thread IDs. These X-GM-* details are Gmail-specific, not universal IMAP features. See Google’s Gmail IMAP extensions documentation.
This protocol explanation does not establish account-specific setup steps, authentication requirements, or eligibility. For connection settings or instructions, consult the current documentation from your email provider and the support information for your email app.
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What IMAP does—and does not—guarantee
- It enables shared mailbox access: multiple clients can reconcile their views against messages and mailbox state held on the server.
- It supports resynchronization: a client can compare cached state with server state after being offline.
- It does not promise identical app behavior: offline storage, refresh timing, notifications, and download policies depend on provider and client implementations.
- It does not submit outgoing mail: sending is a separate protocol function.
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