What are the three major components of electronic mail?
1) User agents (mail readers), 2) Mail servers, 3) SMTP protocol.
What is a user agent in email?
Software used to compose, send, and read mail (e.g., Gmail, Outlook, Yahoo Mail).
What does a mail server do?
Stores incoming messages in user mailboxes and maintains a queue for outgoing mail.
What protocol is used for email transfer between mail servers?
SMTP (Simple Mail Transfer Protocol).
Which TCP port does SMTP use?
Port 25.
What is the relationship between user agent, mail server, and SMTP?
User agents send messages to mail servers via SMTP, which then transfer mail between servers.
Describe the basic email delivery scenario using SMTP.
1) Alice composes a message to Bob. 2) Her UA sends it to her mail server. 3) The SMTP client connects to Bob’s mail server. 4) Message sent via TCP. 5) Message stored in Bob’s mailbox. 6) Bob’s UA retrieves it.
What transport protocol does SMTP run over?
TCP.
Give a sample SMTP interaction.
220 hill.com ready; HELO town.com; MAIL FROM: jack@town.com; RCPT TO: jill@hill.com; DATA; message text; ‘.’; QUIT; 221 hill.com closing connection.
How is an SMTP session terminated?
With the QUIT command followed by a 221 response indicating the connection is closing.
Is SMTP push or pull?
SMTP is a push protocol; the client pushes messages to the receiving server.
How does SMTP differ from HTTP in message handling?
SMTP pushes multiple objects in one multipart message; HTTP pulls each object in its own response.
What standard defines the format of email messages?
RFC 822 defines text message format.
What are the two main parts of an email message per RFC 822?
Header and body, separated by a blank line.
List common header fields in an email.
To:, From:, Subject:.
What character encoding is used in standard SMTP messages?
ASCII text only (7-bit).
What is MIME and why was it introduced?
Multipurpose Internet Mail Extensions — allows transmission of multimedia (images, audio, etc.) in email.
What RFCs define MIME?
RFC 2045 and RFC 2056.
What additional headers are introduced by MIME?
MIME-Version, Content-Type, and Content-Transfer-Encoding.
What does ‘Content-Type’ specify in MIME?
The type and subtype of the attached data (e.g., image/jpeg).
What does ‘Content-Transfer-Encoding’ specify?
The encoding method used (e.g., base64).
What is the purpose of mail access protocols?
They allow users to retrieve email from mail servers.
List the main mail access protocols.
POP3 (Post Office Protocol), IMAP (Internet Mail Access Protocol), and HTTP-based webmail.
Which port does POP3 use?
TCP port 110 (secure: 995).
