{"id":80238,"date":"2024-05-30T18:41:36","date_gmt":"2024-05-30T18:41:36","guid":{"rendered":"https:\/\/mailrelay.com\/glossary\/smtp\/"},"modified":"2026-10-07T00:47:11","modified_gmt":"2026-10-07T00:47:11","slug":"smtp","status":"publish","type":"glossary","link":"https:\/\/mailrelay.com\/en\/glossary\/smtp\/","title":{"rendered":"SMTP"},"content":{"rendered":"\n

SMTP (Simple Mail Transfer Protocol<\/em>) is the standard protocol for sending emails<\/strong> from a client or application to a mail server and transferring them between servers until they reach their destination.<\/p>\n\n

In short: SMTP handles sending email, not reading it or storing it in the inbox<\/strong>. Protocols such as IMAP<\/a> or POP3 are used to check and sync received messages.<\/p>\n\n

<\/p>\n\n

SMTP at a glance<\/h2>\n\n
Question<\/th>Short answer<\/th><\/tr><\/thead>
What does SMTP stand for?<\/td>Simple Mail Transfer Protocol<\/em>.<\/td><\/tr>
What is it used for?<\/td>To send and relay emails.<\/td><\/tr>
Does it receive emails?<\/td>Not directly. Email is usually retrieved using IMAP or POP3.<\/td><\/tr>
Which port does it use?<\/td>Port 25 for server-to-server transfer; ports 587 or 465 are usually used for authenticated sending from clients and applications. Port 2525 can be used as an alternative, depending on the provider.<\/td><\/tr>
Does SMTP encrypt messages?<\/td>Not on its own. The connection can be secured with TLS, either via STARTTLS or implicit TLS.<\/td><\/tr>
Does SMTP prevent spam?<\/td>Not on its own. It must be complemented by authentication, sending policies and technologies such as SPF, DKIM and DMARC.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n

How does SMTP work?<\/h2>\n\n

SMTP works through a dialogue between a client and one or more servers. The usual process is as follows:<\/p>\n\n

\"How<\/figure>\n\n
    \n
  1. Creating the message.<\/strong> The user, an online store, a CRM or an application generates the email.<\/li>\n\n\n\n
  2. Connecting to the SMTP server.<\/strong> The client connects to the outgoing server using a specific host and SMTP port<\/a>.<\/li>\n\n\n\n
  3. Authentication and encryption.<\/strong> When the service requires it, the sender identifies themselves with their credentials and the connection is secured with TLS.<\/li>\n\n\n\n
  4. Identifying the recipient.<\/strong> The server looks up the recipient’s domain and finds the server responsible for receiving its email.<\/li>\n\n\n\n
  5. Transferring the message.<\/strong> The email may pass through one or more SMTP servers or relay hosts<\/a> before reaching the destination server.<\/li>\n\n\n\n
  6. Checks by the receiving server.<\/strong> The receiving server checks the domain authentication, the sender’s reputation and its spam filters.<\/li>\n\n\n\n
  7. Delivery and retrieval.<\/strong> If the message is accepted, it becomes available in the mailbox. The recipient reads it via webmail, IMAP or POP3.<\/li>\n<\/ol>\n\n

    SMTP uses a model based on storing and forwarding<\/strong> messages (store and forward<\/em>). If the destination server is unavailable, the sending server can temporarily keep the message in a queue and retry delivery. The final outcome will depend on the cause of the failure and the provider’s retry policy.<\/p>\n\n

    A simple example of an SMTP send<\/h2>\n\n

    Imagine an online store sends a purchase confirmation:<\/p>\n\n

      \n
    1. The store generates the message for customer@example.com.<\/li>\n\n\n\n
    2. The application hands the email over to its authenticated SMTP service.<\/li>\n\n\n\n
    3. The SMTP service finds the server that handles email for example.com.<\/li>\n\n\n\n
    4. Both servers negotiate the transfer.<\/li>\n\n\n\n
    5. The receiving server runs its checks and accepts or rejects the message.<\/li>\n\n\n\n
    6. If it is accepted, the email appears in the user’s corresponding inbox.<\/li>\n<\/ol>\n\n

      This flow is used both for personal communications and for transactional emails<\/a>, such as order confirmations, shipping notifications or password resets.<\/p>\n\n

      Main SMTP commands<\/h2>\n\n

      During the connection, the client and the server exchange commands and response codes. These are some of the most common:<\/p>\n\n

      Command<\/th>Role<\/th><\/tr><\/thead>
      EHLO<\/td>Starts the communication and announces that the client supports extended SMTP (ESMTP).<\/td><\/tr>
      HELO<\/td>Starts a basic SMTP session when ESMTP is not used.<\/td><\/tr>
      STARTTLS<\/td>Requests upgrading the existing connection to one secured with TLS.<\/td><\/tr>
      AUTH<\/td>Starts client authentication when the server requires it.<\/td><\/tr>
      MAIL FROM<\/td>Specifies the sender’s return address in the SMTP envelope.<\/td><\/tr>
      RCPT TO<\/td>Specifies one of the message’s recipients.<\/td><\/tr>
      DATA<\/td>Starts the transfer of the email’s headers and body.<\/td><\/tr>
      QUIT<\/td>Closes the SMTP session.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n

      The SMTP envelope data does not always match the visible From and To fields of the message. This difference matters when investigating bounces, forwarding or possible spoofing attempts.<\/p>\n\n

      SMTP ports: 25, 587, 465 and 2525<\/h2>\n\n

      The right port depends on whether the message is being transferred between servers or sent from a client or application.<\/p>\n\n

      Port<\/th>Typical use<\/th>Security and recommendation<\/th><\/tr><\/thead>
      25<\/strong><\/td>Transfer or relay between mail servers.<\/td>It is the standard port for server-to-server communication. Some internet service providers block outgoing traffic on it from clients to reduce spam.<\/td><\/tr>
      587<\/strong><\/td>Sending or submission<\/em> from clients and applications.<\/td>It is the standard port for authenticated sending. It normally uses STARTTLS when the provider supports it.<\/td><\/tr>
      465<\/strong><\/td>Authenticated sending with implicit TLS.<\/td>The encrypted connection is established from the start. Use it when your SMTP provider says so.<\/td><\/tr>
      2525<\/strong><\/td>Alternative sending port offered by some providers.<\/td>It is not the standard submission<\/em> port, but it can be useful when other ports are blocked.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n
      \"Which<\/figure>\n\n

      Don’t pick a port at random. Always check your provider’s connection details and confirm three things: port, encryption method and authentication requirement<\/strong>.<\/p>\n\n

      SMTP, IMAP and POP3: the differences<\/h2>\n\n

      SMTP, IMAP and POP3 are all involved in email, but they don’t do the same job.<\/p>\n\n

      Protocol<\/th>Main function<\/th>Direction<\/th>Typical behavior<\/th><\/tr><\/thead>
      SMTP<\/strong><\/td>Send and relay emails.<\/td>Outgoing.<\/td>Delivers the message to the next server along the route.<\/td><\/tr>
      IMAP<\/strong><\/td>Retrieve and sync email from the server.<\/td>Incoming.<\/td>Keeps messages on the server and syncs changes across devices.<\/td><\/tr>
      POP3<\/strong><\/td>Download messages from the server.<\/td>Incoming.<\/td>Downloads email to the device and may delete the copy on the server, depending on the settings.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n

      So an account can use SMTP to send and, at the same time, IMAP to read and sync received messages.<\/p>\n\n

      What is an SMTP server?<\/h2>\n\n

      An SMTP server is the system that accepts, processes, queues and relays outgoing messages. It may belong to an email provider, a company or a specialized SMTP service<\/a>.<\/p>\n\n

      A professional service usually adds capabilities that the basic protocol doesn’t provide on its own:<\/p>\n\n