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E-mail components and message delivery path

Short answer

Key components: user agent (MUA), mail server (MTA/MDA), and protocols SMTP (push), POP3/IMAP (pull). A message travels: sender's UA to source MTA via SMTP/HTTP, DNS MX lookup, SMTP to destination MTA, then retrieval by receiver via POP3/IMAP.

Discuss the e-mail system components with its functions and the path of e-mail messages to be delivered from source domain to destination domain.

Components of the e-mail system

Internet e-mail has three major parts: user agents, mail servers and the mail protocols (SMTP, POP3, IMAP). In more detail, the components are:

  1. User Agent (UA / MUA): the program the user works with (Outlook, Thunderbird, Gmail web interface). It composes, edits, reads, replies to, forwards and stores messages locally.
  2. Mail server (MTA + mailboxes): holds each user's mailbox (incoming messages) and an outgoing message queue. It acts as an SMTP server when receiving and as an SMTP client when forwarding. If delivery fails, it retries (typically every 30 min for several days) and then sends a failure notice.
  3. Message Transfer Agent (MTA): moves mail between servers using SMTP (port 25; submission port 587).
  4. Mail Delivery Agent (MDA): puts the message into the receiver's mailbox.
  5. Mail access protocols: POP3 (port 110, download and delete), IMAP (port 143, keeps mail on the server and manages folders) or HTTP for web-mail. SMTP is a push protocol; POP3/IMAP are pull protocols.
  6. DNS: the MX record of the destination domain tells the sending server which mail server accepts mail for that domain.

Path of a message from source to destination domain

Example: alice@source.com sends to bob@dest.com.

 Alice's UA        Source mail        Dest mail        Bob's UA
 (source.com)      server (MTA)       server (MTA)     (dest.com)
    |  1.SMTP         |                   |                |
    |---------------->|  2.queue          |                |
    |                 |  3. DNS: MX of    |                |
    |                 |     dest.com      |                |
    |                 |  4. TCP+SMTP      |                |
    |                 |------------------>| 5. MDA -> Bob's|
    |                 |                   |    mailbox     |
    |                 |                   |  6. POP3/IMAP  |
    |                 |                   |<---------------|
  1. Alice composes the message in her UA and clicks Send. The UA sends it to her own mail server using SMTP (or HTTP for web-mail).
  2. The source server places the message in its outgoing queue.
  3. The server asks DNS for the MX record of dest.com and then for the IP address of that mail server.
  4. The source server opens a TCP connection (port 25) to the destination mail server and runs the SMTP dialogue: HELO, MAIL FROM, RCPT TO, DATA, QUIT. SMTP transfer is direct: normally there are no intermediate servers.
  5. The destination server accepts the message and its MDA stores it in Bob's mailbox.
  6. When Bob opens his UA, it retrieves the message with POP3 or IMAP (or HTTP for web-mail).

So the protocol chain is: UA -> SMTP -> source MTA -> SMTP -> destination MTA -> POP3/IMAP -> Bob's UA.

✓ Answer checked by Claude Opus 5.5 (high effort)

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