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Simple Mail Transfer Protocol (SMTP). CS-328 Dick Steflik. SMTP RFCs. RFCs - 821, 1870, 1891 Related RFCs RFC 822 (Mail) RFC 1521 (MIME) RFC - 1725 (POP3) RFC - 1730 (IMAP4) Well known port : 25 (TCP) . SMTP Process. Contact server on well known port ASCII oriented command/response

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smtp rfcs
SMTP RFCs
  • RFCs - 821, 1870, 1891
  • Related RFCs
    • RFC 822 (Mail)
    • RFC 1521 (MIME)
    • RFC - 1725 (POP3)
    • RFC - 1730 (IMAP4)
  • Well known port : 25 (TCP)
smtp process
SMTP Process
  • Contact server on well known port
  • ASCII oriented command/response
  • methodology
    • contact server
    • wait for greeting
    • send HELO
    • wait for response
    • send command
    • wait for response
    • repeat last two
smtp commands
SMTP Commands

HELO <sp> <domain><crlf>

MAIL <sp>FROM:<reverse path><crlf>

RCPT <sp>TO:<forward path><crlf>

DATA<crlf> terminates with <crlf>.<crlf>

RSET<cflf>

SEND<sp>FROM:<reverse path><crlf>

SOML<sp>FROM:<reverse path><crlf>

SAML<sp>FROM:<reverst path><crlf>

VRFY<sp><string<crlf>

EXPN<sp> <string><crlf>

HELP<sp><string><crlf>

NOOP<crlf>

QUIT<cflf>

TURN<cflf>

commands
Commands
  • HELP
    • identifies the client to the server, fully qualified domain name, only sent once per session
  • MAIL
    • initiate a message transfer, fully qualified domain of originator
  • RCPT
    • follows MAIL, identifies an addressee, typically the fully qualified name of the addressee
    • for multiple addressees use one RCPT for each addressee
  • DATA
    • send data line by line
    • <cr>.<cr> tells server data transfer is over
commands1
Commands
  • RSET
    • tells server to abort current message and clear all of it buffers
    • same state as after HELO
  • SEND , SOML , SAML
    • like MAIL, outdated not used any more
  • VRFY
    • ask server to verify a user name
    • server replies positively of it knows user, negatively if not
  • EXPN
    • ask server to confirm mailing list alias
    • server reply is multi-line, one per user
commands2
Commands
  • HELP
    • ask server for help
      • by itself get a list of server supported commands
      • <string> get help for that command
  • NOOP
    • ask server to respond with a positive reply
  • QUIT
    • tell server that client is ending session
    • server replies positively and closes connection
  • TURN
    • reverse roles of client and server
      • outdated, rarely used on modern internet
reply codes
Reply codes
  • 211 - System status or help ready
  • 214 - Help message
  • 220 - <domain> Service ready
  • 221 - <domain> Service closing transmission channel
  • 250 - Requested mail action OK, ready
  • 251 - User not local, will forward to <forward path>
  • 354 - Start mail input; end with <crlf>.<crlf>
  • 421 - <domain> Service not avail, closing transmission channel
  • 450 - Requested mail action not taken, mailbox not available
  • 451 - Requested action aborted, local error
  • 452 - Requested action not taken, insufficient storage
  • 500 - Syntax error, command unrecognized
  • 501 - Syntax error in parameters
  • 502 - Command not implemented
  • 503 - Bad sequence of commands
  • 504 - Command Parameter not implemented
reply codes more
Reply codes (more)
  • 550 - Requested action not taken, mailbox unavailable
  • 551 - User not local, please try <forward path>
  • 552 - Requested mail action not taken; exceeded storage allocation
  • 553 - Requested action not taken, mailbox name not allowed
  • 554 - Transaction failed