There are 3 major components to email:

• User agents
  • used on machines to write/send/receive emails
  • e.g. Outlook, mail client
• Mail servers
  • Maintains inbox/output - mailbox with incoming messages for user, message queue of outgoing mail messages
• Simple mail transfer protocol SMTP)
  • Used between mail servers to send email messages
  • Client in this case can be considered the sending mail server, server is receiving mail server

SMTP (RFC 5321)

• Protocol uses TCP to reliably transfer email message from client (mail server initiating connection) to server via port 25

  • A direct transfer is done from the sending server to the receiving server

• 3 phases of transfer:

  • SMTP handshaking (greeting)

  • SMTP transfer of messages

  • SMTP closure

    

• We have command/response interaction (like HTTP)

  • Commands consist of ASCII text
  • Response consists of status code + phrase

• e.g. see example of SMTP interaction

Comparing SMTP with HTTP:

• Both are application layer protocols which use TCP
• Both have ASCII command/response interaction + status codes
• HTTP involves client pull (receive object from server), SMTP involves client push (client pushes to server)
• HTTP has each object encapsulated in its own response message, SMTP has multiple objects sent in multipart message
• SMTP uses persistent connections
• SMTP requires message (header + body) to be in 7-bit ASCII