Email architecture (context)

Email uses three kinds of agents:

• UA (User Agent): the mail client (Outlook, Thunderbird, Gmail app) used to compose and read mail.
• MTA (Message Transfer Agent): the server software that transfers mail between servers; it uses SMTP.
• MAA (Message Access Agent): lets the recipient retrieve mail from the mailbox server; it uses POP3 or IMAP4.

Sender UA --SMTP--> Sender's MTA --SMTP--> Receiver's MTA --> Mailbox
                                                                 |
Receiver UA <------------- POP3 / IMAP4 (MAA) -------------------+

SMTP vs POP

Basis	SMTP (Simple Mail Transfer Protocol)	POP3 (Post Office Protocol v3)
Purpose	Sending / transferring mail	Retrieving mail from the server
Type	Push protocol	Pull protocol
Direction	Client → server, and server → server	Server → client
Used by	MTA (sender UA to server, between servers)	MAA (receiver UA)
Port	25 (587 for submission; 465 secure)	110 (995 with SSL/TLS)
Commands	HELO/EHLO, MAIL FROM, RCPT TO, DATA, QUIT	USER, PASS, STAT, LIST, RETR, DELE, QUIT
Message storage	Only relays it toward the destination	Downloads to the local machine, usually deleting it from the server
Format handling	Transfers 7-bit ASCII (binary content via MIME)	Retrieves the stored message
Connection	TCP, persistent for multiple messages	TCP, one session per download

Mail Access Agents (MAA)

SMTP only pushes mail to the receiver's mail server. Because the recipient's computer is often not always online, the message is stored in the mailbox on the server, and the user's UA fetches it later with a Mail Access Agent. MAAs are pull protocols.

1. POP3 (Post Office Protocol, version 3)

• Simple protocol with three phases: authorization (USER, PASS), transaction (STAT, LIST, RETR, DELE), and update (QUIT applies deletions).
• Two modes: delete mode (messages removed from the server after download) and keep mode (copies remain on the server).
• Advantages: simple, quick, works offline after download, saves server space.
• Disadvantages: poor for several devices (mail lives on one computer), no server-side folders, limited management on the server.

2. IMAP4 (Internet Message Access Protocol, version 4)

• More powerful: mail stays on the server, and the client works on it remotely.
• Features: create, delete, and rename server-side mailboxes/folders; partial download (read headers only, or part of a message); search on the server; flags (read, replied, deleted) synchronised across devices; multiple clients see the same mailbox.
• Port 143 (993 with SSL/TLS).
• Disadvantages: needs more server storage and resources and a continuous connection for full use.

3. Web-based mail (HTTP)

• Gmail, Yahoo, or Outlook.com: the user reads mail through a browser over HTTP/HTTPS; the web server acts as the MAA using POP3/IMAP internally. Mail is sent from the browser to the mail server by HTTP, then between servers by SMTP.

POP3 vs IMAP4

Basis	POP3	IMAP4
Mail location	Downloaded to the client	Kept on the server
Multi-device use	Poor	Excellent
Server-side folders	No	Yes
Partial fetch / search	No	Yes
Complexity and server load	Low	Higher

Summary: SMTP is used to send mail; POP3 and IMAP4 (the Mail Access Agents) are used to read mail.