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Email Protocols: SMTP, POP3, and IMAP

Learn how SMTP sends email, how POP3 retrieves it, and how IMAP synchronizes a server-based mailbox across devices.

Email protocols are standardized rules that let mail clients and mail servers exchange messages. In a basic email system, SMTP handles sending and relaying mail, while POP3 and IMAP provide ways for a user to access received messages.

The most important distinction is between sending a message and accessing an existing mailbox. A mail client may use SMTP to submit an outgoing message, then use POP3 or IMAP to retrieve messages that are already stored on a mail server.

Key Terms

  • Mail client: An application used to read, organize, send, and retrieve email.
  • Mail server: A system that accepts, relays, stores, and provides access to email.
  • Remote mailbox: A user's message store hosted on an email server.
  • TCP port: A numbered endpoint that identifies a network service on a host.
  • Message retention: Whether messages remain on the server after a client retrieves them.
  • Push protocol: A communication model in which the sending system begins the transfer.
  • Pull protocol: A communication model in which the receiving client requests data from a remote system.

How Email Protocols Work Together

Sending and receiving are separate operations. When a user writes a message, the mail client submits it through SMTP. SMTP can then relay the message from the sender's mail server toward the recipient's mail server. The recipient's client later accesses the recipient's remote mailbox through POP3 or IMAP.

Basic Email Message Flow

Stage — Participating systems — Protocol — Outcome

Outgoing message submission — Mail client and sender's mail server — SMTP — The client submits the message for delivery.

Mail server relay — Sender's mail server and recipient's mail server — SMTP — Mail servers transfer the message toward the destination mailbox.

Recipient mailbox access through POP3 — Recipient's client and recipient's mail server — POP3 — The client downloads messages, which may then be removed from the server.

Recipient mailbox access through IMAP — Recipient's client and recipient's mail server — IMAP — The client accesses messages that generally remain on the server.

SMTP: Sending and Relaying Email

SMTP stands for Simple Mail Transfer Protocol. It is used to submit outgoing messages and to relay messages between mail servers.

SMTP is push-oriented: the sending side starts the delivery. For example, when a user selects Send, the mail client or an email submission service starts an SMTP connection and presents the outgoing message to the sender's mail server.

Typical SMTP Message Path

  1. A user composes a message in a mail client.
  2. The client submits the message to the sender's mail server using SMTP.
  3. The sender's mail server uses SMTP to relay the message toward the recipient's mail server.
  4. The recipient's mail server accepts and stores the message in the recipient's remote mailbox.
  5. The recipient later accesses that mailbox using POP3 or IMAP.

The traditional server-to-server SMTP service uses TCP port 25. SMTP is not normally the protocol a user uses to browse and download a received mailbox. It is primarily the protocol for outgoing submission and mail-server delivery.

POP3: Downloading Messages

POP3 stands for Post Office Protocol version 3. It lets an email client collect messages from a remote mailbox.

POP3 is pull-oriented: the receiving client starts the connection and requests messages from the server. Its conventional model is download-focused. A client downloads new messages to local storage and may remove the server copies after retrieval.

The traditional POP3 service uses TCP port 110.

Server Copy Retention

A POP3 client can usually be configured to leave a copy of each message on the server. This allows another client to download the same messages later. However, retaining copies does not turn POP3 into a synchronized mailbox system. Read status, folders, and other mailbox changes are not inherently shared between clients as they are with IMAP.

If a desktop client downloads and removes messages, the desktop may become the primary message store. A phone connecting later may find no messages because the server copies have already been deleted.

POP3 Example

A user has one desktop computer and wants email downloaded for local use. The desktop connects through POP3, retrieves new messages, and deletes the server copies. This is a simple local-download arrangement, but it is a poor fit when the same mailbox must be used consistently from several devices.

IMAP: Accessing a Synchronized Mailbox

IMAP stands for Internet Message Access Protocol. It gives mail clients access to messages that generally remain stored on the mail server.

IMAP is also pull-oriented: the client initiates mailbox access and requests information from the server. Unlike the traditional download-and-remove POP3 model, IMAP treats the server as the main location for the mailbox.

The traditional IMAP service uses TCP port 143.

Server-Side Mailbox State

Because messages remain on the server, multiple devices can access the same mailbox. IMAP can synchronize mailbox information such as whether a message is read and how messages are organized into folders. If a user reads a message on a phone, a laptop using the same IMAP mailbox can generally see that changed state.

Users normally remove IMAP messages only when they deliberately delete them. Until then, the server remains the central copy of the mailbox.

IMAP Example

A user opens the same account on a laptop and a phone through IMAP. Both devices access the server-resident messages and mailbox organization, so the devices can reflect a shared view of the account.

SMTP, POP3, and IMAP Comparison

Protocol — Primary purpose — Connection initiation model — Typical message location after access — Traditional TCP port — Best-fit use case

SMTP — Submit outgoing email and relay it between mail servers — Push; the sending side starts delivery — The message is delivered to the destination server's mailbox — 25 — Sending and server-to-server delivery

POP3 — Retrieve email from a server through downloading — Pull; the client requests messages — Often local after download; server copies may be removed unless retention is enabled — 110 — Simple local downloading, especially for a single primary device

IMAP — Access and synchronize a remote mailbox — Pull; the client requests mailbox data — Primarily on the server — 143 — Mailbox access from multiple devices

POP3 Versus IMAP

POP3 and IMAP both let a client retrieve received email, but they use different mailbox models.

  • POP3: Download-oriented. Messages are commonly copied to the client and may be removed from the server.
  • IMAP: Server-mailbox-oriented. Messages remain on the server and clients synchronize access and mailbox state.
  • POP3 with retained copies: Other devices may download messages later, but folders and read status are not inherently synchronized.
  • IMAP: Better suited to a laptop, phone, tablet, and webmail interface sharing one mailbox.

Which Protocol Should You Choose?

  • Choose SMTP whenever the task is sending or relaying email.
  • Choose POP3 when simple local downloading is the goal and one device is the primary place for storing messages.
  • Choose IMAP when the same mailbox must be available consistently across multiple devices.

In a typical account, SMTP works together with either POP3 or IMAP: SMTP handles outgoing delivery, while POP3 or IMAP handles access to incoming mail.

Practical Troubleshooting

Receiving Works, but Sending Fails

Receiving and sending use different protocol roles. A client may successfully access mail through POP3 or IMAP while having a problem with SMTP. Check that the outgoing SMTP service is configured and that the client is using the intended SMTP port.

Messages Appear on a Desktop but Not on a Phone

Check whether the desktop uses POP3 and removes server copies after downloading. If it does, the phone cannot retrieve messages that no longer exist in the remote mailbox. IMAP is generally more appropriate when several devices need access to one mailbox.

Email Looks Different on Multiple Devices

Determine whether the account uses POP3 or IMAP. POP3 clients can maintain separate local copies and may not share read state or folders. IMAP is designed to maintain a shared server-side mailbox state.

A Connection to the Expected Service Fails

First confirm that the selected protocol matches the intended action: SMTP for sending, or POP3 or IMAP for retrieving mail. Then check the traditional default TCP ports: 25 for SMTP, 110 for POP3, and 143 for IMAP.

Summary

  • SMTP means Simple Mail Transfer Protocol and sends or relays outgoing email. Its traditional port is TCP 25.
  • POP3 means Post Office Protocol version 3 and commonly downloads messages to a client. Its traditional port is TCP 110.
  • IMAP means Internet Message Access Protocol and provides access to a mailbox that remains primarily on the server. Its traditional port is TCP 143.
  • SMTP is push-oriented because the sending side starts delivery.
  • POP3 and IMAP are pull-oriented because the receiving client requests mailbox data.
  • POP3 may remove server copies after download; IMAP normally keeps messages server-side until the user deletes them.
  • For multi-device access, IMAP is usually the better fit.

Continue with email protocol fundamentals when reviewing the roles and differences of these services.