Email, SMTP, IMAP

Email

Three major components:

• user agents
• mail servers
• simple mail transfer protocol: SMTP

User Agent

• aka “mail reader”
• composing, editing, reading mail messages
• e.g., Outlook, iPhone mail client
• outgoing, incoming messages stored on server

E-mail: mail servers

mail servers:

• mailbox contains incoming messages for user
• message queue of outgoing (to be sent) mail messages

SMTP protocol between mail servers to send email messages

• client: sending mail server
• ”server”: receiving mail server

SMTP RFC (5321)

• uses TCP to reliably transfer email message from client (mail server initiating connection) to server, port 25
  • direct transfer: sending server (acting like client) to receiving server
• 3 phases of transfer
  • SMTP handshaking (greeting)
  • SMTP transfer of messages
  • SMTP closure
• command/response interaction (like HTTP)
  • commands: ASCII text
  • response: status code and phrase

Example:

SMTP vs HTTP

• HTTP: client pull

• SMTP: client push

• both have ASCII command/response interaction, status codes

• HTTP: each object encapsulated in its own response message

• SMTP: multiple objects sent in multipart message

• SMTP uses persistent connections

• SMTP requires message (header & body) to be in 7-bit ASCII

• SMTP server uses CRLF.CRLF to determine end of message

Mail message format

• header
• body
  • ASCII characters only

Retrieving email: mail access protocols

• SMTP: delivery/storage of e-mail messages to receiver’s server $\to$ used 2x
• mail access protocol: retrieval from server
  • IMAP: Internet Mail Access Protocol [RFC 3501]: messages stored on server, IMAP provides retrieval, deletion, folders of stored messages on server
• HTTP: gmail, Hotmail, Yahoo! Mail, etc. provides web-based interface on top of SMTP (to send), IMAP (or POP) to retrieve e-mail messages