POP3 vs IMAP4 and Delivery of E-mail

Part A — POP3 vs IMAP4

Aspect	POP3 (RFC 1939)	IMAP4 (RFC 3501)
Full form	Post Office Protocol version 3	Internet Message Access Protocol version 4
Basic model	Download-and-delete (default) or download-and-keep	Messages remain on the server; the client manipulates the server's copy
Server-side storage	Mailbox is emptied after retrieval → mail stored on one machine	All mail retained on the server; folders, flags and read state kept server-side
Client access	Effectively one client at a time (mailbox is locked)	Many clients/devices can access the same mailbox simultaneously
Operations	Simple: USER, PASS, LIST, RETR, DELE, STAT, QUIT	Rich: LOGIN, SELECT, FETCH, STORE, SEARCH, CREATE/DELETE, EXPUNGE, NOOP
Folders	Not supported (flat mailbox)	Multiple folders supported, created/deleted on the server
Partial download	No — a whole message must be fetched	Yes — headers only, byte ranges, individual MIME parts
Server-side search	No	Yes (SEARCH)
Default ports	110 (cleartext), 995 (TLS)	143 (cleartext), 993 (TLS)
Typical use	Simple offline reading, low server load	Webmail, smartphones, sync across devices
Authentication	USER/PASS (cleartext unless TLS)	LOGIN or SASL (CRAM-MD5, OAuth2, …)

Both protocols are used only to retrieve mail from the user's own mailbox; moving a message between hosts is always done by SMTP.

Part B — How Alice's message reaches Bob

Application-layer protocols used, in order: HTTPS (HTTP) → DNS → SMTP (possibly several hops) → POP3 or IMAP4.

1. Alice composes the message in her web-based account (Gmail/Hotmail). Since she uses a browser, the message is uploaded to her provider's web server using HTTP over TLS (HTTPS), port 443. (With an installed mail client she would instead submit it with SMTP on port 587.)
2. Alice's mail server (originating MTA) takes the message. Before sending, it queries DNS for Bob's domain MX record (then an A/AAAA record) to learn the address of Bob's mail exchanger — mail cannot be delivered without this lookup.
3. SMTP transfer: Alice's server opens a TCP connection to Bob's mail server on port 25 and runs SMTP — HELO/EHLO → MAIL FROM → RCPT TO → DATA → QUIT — handing over the envelope and the message. If Bob's server is down, the message is queued and retried.
4. Possible relays: the message may pass through one or more intermediate mail servers, each forwarding it with SMTP, until it reaches Bob's mail server (destination MTA), which accepts it and appends the message to Bob's mailbox.
5. Bob retrieves the mail with a mail client that logs into his own server using POP3 (110/995) or IMAP4 (143/993): USER/PASS (or LOGIN), LIST, then RETR (POP3) or FETCH (IMAP4), followed by QUIT. Under POP3 the message is normally deleted from the server after download; under IMAP4 it stays on the server.

 Alice's host        Alice's mail server      DNS server        Bob's mail server     Bob's host
 (web browser)       (webmail provider)                          (Bob's domain)        (mail client)
      |                      |                     |                     |                   |
      | 1 HTTPS (443)        |                     |                     |                   |
      |   compose + send     |                     |                     |                   |
      |--------------------->|                     |                     |                   |
      |                      | 2 DNS query: MX for bob.com              |                   |
      |                      |-------------------->|                     |                   |
      |                      | 3 MX answer: mail.bob.com                |                   |
      |                      |<--------------------|                     |                   |
      |                      | 4 SMTP (25): EHLO / MAIL FROM / RCPT TO / DATA               |
      |                      |---------------------------------------->|                   |
      |                      |                     |                     | 5 message stored  |
      |                      |                     |                     |   in Bob's mailbox|
      |                      |                     |                     |                   |
      |                      |                     |                     | 6 POP3 (110/995)  |
      |                      |                     |                     | or IMAP4 (143/993)|
      |                      |                     |                     |<------------------|

Series of application-layer protocols: HTTP/HTTPS (Alice → her provider), DNS (MX lookup), SMTP (Alice's server → Bob's server, possibly several relays), and POP3 or IMAP4 (Bob's server → Bob's client) — all carried over TCP/IP beneath them.