Electronic Mail (SMTP, POP, IMAP, MIME)

Context: FIT1047_MOC · the second Application-layer case study · asymmetric design: SMTP pushes mail toward the destination server, POP/IMAP pull it to the reader

Quick Revision

• 🎯 Objective: map the journey ➔ sender’s client —SMTP→ sender’s server —SMTP→ recipient’s server —POP/IMAP→ recipient’s client.
• 📦 Core Components: SMTP dialogue (HELO → MAIL FROM → RCPT TO → DATA → . → QUIT) ➔ POP vs IMAP retrieval ➔ MIME for anything beyond plain text.
• ⚡ Key Constraint: SMTP carries plain text only — attachments exist because MIME base64-encodes binary into text.

📝 Core

• SMTP ➔ transfers messages client→server AND server→server (e.g. smtp.live.com → smtp.gmail.com).
• POP ➔ download to the client, delete from server — one-device model.
• IMAP ➔ messages stay on the server; multiple clients share one mailbox simultaneously — the modern default.
• MIME ➔ Multi-Purpose Internet Mail Extensions: character sets (Unicode), non-text attachments, multi-part bodies — e.g. Content-Type: image/jpeg + Content-Transfer-Encoding: base64 turning an image into text lines.

📊 Exam Execution Trace — lecture SMTP session (verbatim, server lines numbered)

S: 220 MyMailServer ESMTP …            ← greeting
C: HELO my.laptop
S: 250 MyMailServer Hello laptop …
C: MAIL FROM:<alice@mymail.com>
S: 250 OK
C: RCPT TO:<john@mymail.edu>
S: 250 Accepted
C: DATA
S: 354 Enter message, ending with "." on a line by itself
C: From/To/Date/Subject headers … blank line … body … "."
S: 250 OK id=…
C: QUIT
S: 221 closing connection

• Message anatomy inside DATA ➔ header block (From, To, Date, Subject) + blank line + body — headers here are content, distinct from the SMTP envelope (MAIL FROM/RCPT TO).

⚖️ Core Decision Matrix — POP vs IMAP

Aspect	POP	IMAP
messages live	on the client (server copy deleted)	on the server
multi-device	poor	designed for it (simultaneous clients)
offline habit	classic single PC	phones + laptop + webmail

⚠️ Common Mistakes

• 💡 Envelope ≠ header ➔ MAIL FROM:/RCPT TO: route the mail; the From:/To: inside DATA are mere display text — spoofing exploits exactly this gap.
• 💡 The lone dot terminates ➔ . alone on a line ends DATA; it’s protocol syntax, not punctuation.
• 💡 SMTP both hops ➔ students often say “SMTP = server-to-server only”; it also carries the first hop from the sender’s client.

🧠 Active Recall

Alice (hotmail) mails Bob (gmail). Name the protocol on each of the three hops.

Answer

• Short answer: Alice’s client →SMTP→ smtp.live.com →SMTP→ smtp.gmail.com →POP or IMAP→ Bob’s client.
• Why: Push then pull ➔ SMTP pushes toward the destination server; retrieval waits until Bob’s client asks.

Why does emailing a photo work when SMTP is text-only?

Answer

• Short answer: MIME encodes the binary as base64 text with Content-Type/Content-Transfer-Encoding headers; the receiving client decodes it back.
• Why: Layered compatibility ➔ extending the message format instead of the transfer protocol kept every existing SMTP server working.