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Running mail: SMTP and IMAP

The chain from the client to the submission server, the transfer server, the mailbox and then IMAP or POP3; the SMTP dialogue and its codes, envelope and headers, MX, open relay and the queue.

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  • SMTP: what is it?

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    The protocol that sends mail, from the client to its server then from server to server.

    SMTP is only for sending (port 587 for submission, 25 for transfer). To read a mailbox, IMAP or POP3 is used.

  • What does the server's reply to RCPT TO mean?

    ```text
    S: 220 mail.example.org ESMTP Postfix
    C: EHLO poste.example.org
    S: 250 mail.example.org
    C: MAIL FROM:<lea@example.org>
    S: 250 2.1.0 Ok
    C: RCPT TO:<tom@example.net>
    S: 554 5.7.1 <tom@example.net>: Relay access denied
    `

    • The server refuses to relay to that domain for an unauthorised client
    • Tom's mailbox does not exist on his domain's server, which told the client so
    • The server is overloaded for now, the client must try again a little later
    • The message exceeds the maximum size the server agrees to receive
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    The server refuses to relay to that domain for an unauthorised client

    The domain example.net is not handled by this server, and the client is neither on an allowed network nor authenticated: relaying is refused. A 554 is permanent; an unknown user would give 550 5.1.1.

  • The client submits its message on port …, with authentication, whereas servers transfer mail to each other on port ….

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    The client submits its message on port 587, with authentication, whereas servers transfer mail to each other on port 25.

    Separating submission (authenticated users) from transfer (servers between themselves) makes it possible to block outbound 25 from workstations without stopping anyone from writing.

  • IMAP: what is it?

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    The protocol that reads and files mail directly on the server.

    Messages stay on the server: read or filed from a phone, they are also on the computer. Ports 143, or 993 when encrypted.

  • How a message is delivered: which step comes after "The client hands the message to the submission server, after authenticating"?

    • It passes the message on by SMTP to the recipient's receiving server
    • The server queries the DNS for the recipient's domain MX records
    • The delivery agent drops the message in the recipient's mailbox
    • The recipient reads their mail with IMAP or POP3
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    The server queries the DNS for the recipient's domain MX records

    Mail goes through submission, MX lookup, transfer, delivery, then reading. SMTP covers the first three steps; IMAP and POP3 are only for reading.

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