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Web-mail to POP3 message transfer protocols

Short answer

Alice's browser sends the message to her webmail server via HTTP POST. After DNS MX lookup, that server pushes it to Bob's server using SMTP. Bob's client retrieves it with POP3. Application layer protocols: HTTP, DNS, SMTP, POP3.

Suppose Alice with a web-based email account sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message is transferred from Alice to Bob. Be sure to list the series of application layer protocols that are used to move the message between the hosts.

Setup: Alice uses web-mail (for example Gmail/Yahoo), so her "user agent" is a web browser. Bob has a mailbox on his own mail server and reads it with a POP3 client. Assume Alice's mail server is mail.alice-isp.com and Bob's is mail.bob.com.

 Alice's browser   Alice's mail server   Bob's mail server   Bob's PC
        |  HTTP           |                     |               |
        |---------------->|                     |               |
        |                 |------ SMTP -------->|               |
        |                 |                     |<---- POP3 ----|

Steps of transfer

  1. Alice to her mail server (HTTP). Alice logs in through the browser, composes the message and clicks Send. The browser sends the message with an HTTP POST to her web-mail server. Unlike with a normal mail client, no SMTP is used on this hop.
  2. Queue. The web-mail server places the message in its outgoing queue.
  3. DNS lookup. The server queries DNS for the MX record of bob.com and then the A record of mail.bob.com. This uses another application layer protocol: DNS.
  4. Alice's server to Bob's server (SMTP). The server opens a TCP connection to port 25 of Bob's mail server and sends the message using SMTP (HELO, MAIL FROM, RCPT TO, DATA, QUIT). If Bob's server is down, the message stays in the queue and is retried.
  5. Storage. Bob's mail server puts the message in Bob's mailbox.
  6. Bob retrieves the message (POP3). Bob's mail client connects to port 110 of his server and runs POP3: authorization phase (USER, PASS), transaction phase (LIST, RETR, DELE), and update phase (QUIT). The message is downloaded to Bob's PC and, in the usual "download and delete" mode, removed from the server.

Protocol series

HopProtocol
Alice's browser -> her web-mail serverHTTP (POST)
Name lookup of Bob's mail serverDNS
Alice's mail server -> Bob's mail serverSMTP
Bob's mail server -> Bob's PCPOP3

HTTP, SMTP and POP3 run over TCP; DNS normally runs over UDP. The key point: SMTP is a push protocol used here only between the mail servers; POP3 is a pull protocol used by the receiver; and HTTP replaces SMTP on the sender's side because Alice uses web-mail.

✓ Answer checked by Claude Opus 5.5 (high effort)

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