SMTP

1
SMTP

• Simple Mail Transfer Protocol

2
RFC 821

• Defines SMTP
• Written 1982
• Objective "to transfer mail reliably and
  efficiently"
• Sender establishes two-way communication with
  receiver
• Sender send the MAIL command.
• If receiver can receives, sends the OK command
• Sender specifies the recipient with RCPT command
• Sender sends data with DATA command
• Terminates with special sequence

3
Client Design

• Mail port almost always on port 25 (sometimes
  moved for security reasons)
• Open up socket
• Have threaded reader from socket
• Write commands using correct protocol
• Note ComMAnDs Are NoT CASe SensITiVE

4
HELO

• HELO ltdomaingt
• Identifies the name of the sender
• Replies with OK
• Not checked for accuracy
• Sometimes not required
• Example
• HELO bob.com

5
MAIL

• Used to initiate a mail transaction
• MAIL FROM ltemail_address_of_sendergt
• Once again, not checked for accuracy
• Example
• MAIL FROM bob_at_bob.com

6
RCPT

• Identifies the recipient of the mail
• Multiple recipients are achieved by multiple uses
  of this command
• RCPT TO ltemail_addressgt
• Example
• RCPT TO jonpreston_at_mail.clayton.edu

7
DATA

• Represents the actual data to be sent to the
  receiver
• Terminated with single period on a line by itself
  "ltCRLFgt.ltCRLFgt"
• Can contain
• From
• To
• Subject
• Date
• Reply
• Received

8
QUIT

• It, uh does umm

9
Reply Codes

• 500 Syntax error, command unrecognized This may
  include errors such as command line too long
• 501 Syntax error in parameters or arguments
• 502 Command not implemented
• 503 Bad sequence of commands
• 504 Command parameter not implemented
• 211 System status, or system help reply
• 214 Help message Information on how to use the
  receiver or the meaning of a particular
  non-standard command this reply is useful only
  to the human user
• 220 ltdomaingt Service ready
• 221 ltdomaingt Service closing transmission channel
  
• 421 ltdomaingt Service not available, closing
  transmission channel This may be a reply to any
  command if the service knows it must shut down
• 250 Requested mail action okay, completed
• 251 User not local will forward to
  ltforward-pathgt
• 450 Requested mail action not taken mailbox
  unavailable E.g., mailbox busy
• 550 Requested action not taken mailbox
  unavailable E.g., mailbox not found, no access
• 451 Requested action aborted error in processing
  
• 551 User not local please try ltforward-pathgt
• 452 Requested action not taken insufficient
  system storage
• 552 Requested mail action aborted exceeded
  storage allocation
• 553 Requested action not taken mailbox name not
  allowed E.g., mailbox syntax incorrect

10
Minimum Implementation

• For workability, the following must be recognized
  by the server
• HELO
• MAIL
• RCPT
• DATA
• RSET
• NOOP
• QUIT

11
Goin' Old-School

• A Telnet session provides simple socket
  functionality
• Telnet to a server on port 25 to practice your
  SMTP commands!
• Easy on UNIX, Windows messes up

12
Example
13
NOOP

• Does nothing, other than send back an OK command
• Useful for debugging your sockets

14
VRFY

• Used to verify an account exists
• Can be a security risk

15
EXPN

• Expand and return a list
• Hard to find a server that supports this

16
RSET

• Current mail transaction is to be aborted
• Any information sent is to be discarded
• Receiver must send OK

17
HELPIf you can't remember
18
Different HELP
19
BUSTED
20
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