Simple Mail Transfer Protocol (SMTP)

1
Simple Mail Transfer Protocol(SMTP)

• CS-328
• Dick Steflik

2
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)

3
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

4
SMTP Commands
HELO ltspgt ltdomaingtltcrlfgt MAIL ltspgtFROMltreverse
pathgtltcrlfgt RCPT ltspgtTOltforward
pathgtltcrlfgt DATAltcrlfgt terminates with
ltcrlfgt.ltcrlfgt RSETltcflfgt SENDltspgtFROMltreverse
pathgtltcrlfgt SOMLltspgtFROMltreverse
pathgtltcrlfgt SAMLltspgtFROMltreverst
pathgtltcrlfgt VRFYltspgtltstringltcrlfgt EXPNltspgt
ltstringgtltcrlfgt HELPltspgtltstringgtltcrlfgt NOOPltcrlfgt Q
UITltcflfgt TURNltcflfgt
5
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
• ltcrgt.ltcrgt tells server data transfer is over

6
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• like MAIL, outdated not used any more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

7
Commands

• HELP
• ask server for help
• by itself get a list of server supported commands
• ltstringgt 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• outdated, rarely used on modern internet

8
Reply codes

• 211 - System status or help ready
• 214 - Help message
• 220 - ltdomaingt Service ready
• 221 - ltdomaingt Service closing transmission
  channel
• 250 - Requested mail action OK, ready
• 251 - User not local, will forward to ltforward
  pathgt
• 354 - Start mail input end with ltcrlfgt.ltcrlfgt
• 421 - ltdomaingt 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

9
Reply codes (more)

• 550 - Requested action not taken, mailbox
  unavailable
• 551 - User not local, please try ltforward pathgt
• 552 - Requested mail action not taken exceeded
  storage allocation
• 553 - Requested action not taken, mailbox name
  not allowed
• 554 - Transaction failed