POP (Post Office Protocol — POP3)
1. Definition
POP3 (RFC 1939) is an application-layer protocol by which an e-mail client downloads (pulls) mail from a mail server's mailbox to the local machine. Transport: TCP port 110 (plaintext) or 995 (POP3S over SSL/TLS).
2. Where POP Fits
sender --SMTP--> receiver MTA (mailbox on server)
|
client UA --POP3 (110/995)--> downloads messages --> deletes/marks on server
SMTP pushes mail to the server; POP3 pulls it to the client. The server acts like a temporary post office holding letters until collected.
3. POP3 Session Phases
| Phase | State | What happens |
|---|---|---|
| 1. Authorisation | AUTH | USER name + PASS password (or APOP digest) |
| 2. Transaction | TRANS | Client issues commands: list/stat/retrieve/delete |
| 3. Update | UPDATE | After QUIT, server commits the deletions |
Commands
| Command | Purpose |
|---|---|
USER, PASS |
Login credentials |
STAT |
Maildrop status (message count, total size) |
LIST [n] |
Message numbers and sizes |
RETR n |
Retrieve message n (downloaded) |
DELE n |
Mark message n for deletion (removed on QUIT) |
RSET |
Un-mark all deletions |
NOOP, TOP n m, UIDL |
Keep-alive, header + m lines, unique-ids |
QUIT |
End session, commit changes |
Replies: single-line +OK (success) or -ERR (failure).
4. Example Session
S: +OK POP3 server ready
C: USER student
S: +OK
C: PASS secret
S: +OK mailbox open, 2 messages
C: STAT
S: +OK 2 5120
C: RETR 1
S: +OK 3000 octets ...message text...
C: DELE 1
S: +OK message 1 deleted
C: QUIT
S: +OK 1 message deleted
5. POP3 Features — Download-and-Delete vs Download-and-Keep
| Mode | Behaviour | Effect |
|---|---|---|
| Download-and-delete (default) | Messages removed from server after download | Mail exists on one device only; server stays clean |
Download-and-keep (UIDL-based clients) |
Messages left on server | Multiple devices can see the same mail |
Other characteristics: works offline after download (good for dial-up era, saves server disk), simple and lightweight, no folder structure on the server, one mailbox per session.
6. POP3 vs IMAP
| Aspect | POP3 | IMAP |
|---|---|---|
| Port | 110 / 995 | 143 / 993 |
| Model | Download to client | Mail stays on server; client views sync |
| Server folders | None | Full folder hierarchy |
| Multi-device access | Weak | Designed for it |
| Offline use | Excellent (local copies) | Good (with caching) |
| Server storage | Emptied (usually) | Grows over time |
| Best for | Single-device, minimal server | Modern multi-device access |
7. Security
Plain POP3 sends credentials and mail in clear text; use POP3S (995, TLS) or STLS (STARTTLS extension) and strong passwords; APOP provides MD5-challenge authentication.
8. Exam Points
- Explain POP3 architecture and its three session phases (authorisation, transaction, update) with a command example — 10 marks (very frequent).
- List POP3 commands (USER, PASS, STAT, LIST, RETR, DELE, QUIT) with server replies — 7 marks.
- Compare POP3 and IMAP — 5 marks (frequent).
- Short note: why POP3 is called a pull protocol while SMTP is a push protocol — 3 marks.