SMTP protocol

Simple Mail Transfer Protocol(简单邮件传输协议)

transport protocol - TCP
SMTP - RFC 821
port - 25

• User Agent
  • composes, edits and reads mail messages
  • outgoes and incomes messages stored on server
• Mail Server
  • mailbox: contains incoming messages(yet to be read) for user
  • message: queue of outgoing(to be sent) mail messages
• client/server model
  • client: sending mail server
  • server: receiving mail server

Feature

• direct transfer: sending server to receiving server
• command/response interaction
  • command: ASCII text
  • response: status code and phrase
• messages must be in 7-bit ASCII
• certain character strings aren't permitted in message. Thus message has to be encoded.
• SMTP server uses CRLF
  • CRLF - determine the end of message

Comparsion

	HTTP	SMTP
operation	pull	push
object	each object is encapsulated in its own response message	multiple objects message sent in a multipart message

Mail message format

• RFC 822
• header lines
  • To:
  • From:
  • Subject:
• body
  • the message(ASCII only)

MIME types

• Text
  • plain
  • html
• Image
  • jpeg
  • gif
• Audio
  • basic(8-bit mu-law encoded)
  • 32kadpcm(32 kbps coding)
• Video
  • mpeg
  • quicktime
• Application
  • other data that must be processed by reader before "viewable"
  • example subtypes
    • msword
    • octet-stream

Mail access protocols

• SMTP: delivery/storage to receiver's server
• Mail access protocol: retrival from server
  • POP: Post Office Protocol(RFC 1939)
    • authorization (agent<=>server) and download
  • IMAP: Internet Mail Access Protocol(RFC 1730)
    • more features(more complex)
    • manipulation of stored msgs on server
  • HTTP(Web-Based Email): Hotmail, Yahoo! Mail, etc.

Protocol between mail server&user pc

pop3/IMAP