Email

• Email is a form of asynchronous communication (cf. phone conversation)
• An email system has 3 major components
  • user agents
  • mail servers
  • Simple Mail Transfer Protocol (SMTP)
• Each user is known through name and domain address, and has personal mailbox(es) on the host housing the mail server.
• The user agent (“mail reader” e.g. unix mail, apple mail, etc) allows user to compose, send, reply, forward, read, and delete email messages.

Email, continued

• In the beginning all email messages were in plain text. These days, messages can be html-formatted, and one can send pictures, movies, binary files, etc. through email.
• The mail server is responsible for sending and receiving email messages with other mail servers. It has a queue of messages waiting to be processed.
• The mail server takes care of adding new messages to the recipient’s mailbox.
• Messages sent to invalid recipient will be rejected with a feedback email message to sender.
• Messages sent to valid recipient that can’t be delivered immediatedly are put on the outgoing queue and the sending is retried at regular invervals (like every 30 minutes). If delivery can’t be achieved within a few days, the message is discarded after notifying the sender.

SMTP

• SMTP uses the service of TCP transport protocol to transfer mail. It uses a client-server model. The TCP connection is persistent.
• SMTP is specified in RFC 5321
• The body of email message must be encoded in 7-bit ASCII.

Example SMTP conversation

    S: 220 hamburger.edu
    C: Helo crepes.fr
    S: 250 Hello crepes.fr, pleased to meet you
    C: MAIL FROM: <alice@crepes.fr>
    S: 250 alice@crepes.fr ... Sender ok
    C: RCPT TO: <bob@hamburger.edu>
    S: 250 bob@hamburger.edu ... Recipient ok
    C: DATA
    S: 354 Enter mail, end with "." on a line by itself
    C: Do you like ketchup?
    C: How about pickles?
    C: .
    S: 250 Message accepted for delivery
    C: QUIT
    S: 221 hamburger.edu closing connection

Mail access protocols

• Post Office Protocol, Version 3 (POP3)
• Internet Mail Access Protocol (IMAP)
• Web-based email (HTTP)