Simple Mail Transfer Protocol

Mail RFCs (1)

• SMTP RFC5321 (obsoletes RFC2821)
• SMTP Authentication RFC2554
• SMTP STARTTLS RFC2487
• Message Submission RFC4409

Mail RFCs (2)

• Message Header RFC5322 (obsoletes RFC5322)
• MIME 1 Body Format RFC2045
• MIME 2 Media Types RFC2046
• MIME 3 Header Extension RFC2047
• MIME 4 Registration RFC4289 (旧 RFC2048)
• MIME 5 Examples RFC2049

SMTP Server

• Sendmail
• Postfix

Session Initialion

• C->S Connect TCP 25
• S->C 220 greeting message

Client Initiation

• C->S EHLO Client Identiy
• S->C 250 Server Idenity + ESMTP Capabilities

Mail Transactions (1)

• C->S MAIL From: sender
• S->C 250 Ok
• C->S RCPT To: recipient
• S->C 250 Ok

Mail Transactions (2)

• C->S DATA
• S->C 354 End data with .
• C->S (message)
• C->S .
• S->C 250 Ok

Session Close

• C->S QUIT
• S->C 221 Bye

Debugging Commands

• EXPN ... Expand (list address)
• VRFY ... Verify (e-mail address)
• almost always disabled

SMTP Relay

• Relay to internal SMTP servers
• Relay to non-IP email system
  • Historical ... @BITNET
  • X.400 MHS
• From internal users to the Internet

Received headers

• When forwarding a message, a gateway MUST prepend a Received: line
• but it MUST NOT alter in any way a Received: line that is already in the header section.

Other SMTP Commands

• HELO ... Older EHLO
• RSET (reset)
• HELP/NOOP

SMTP Response Code (1)

• 2yz ... Positive Completion reply
• 3yz ... Positive Intermediate reply
• 4yz ... Transient Negative Completion reply
• 5yz ... Permanent Negative Completion reply

SMTP Response Code (2)

• x0z Syntax related
• x1z Informational
• x2z Connections related
• x5z Mail system related

Reply continuation

• Example:

     250-First line
     250-Second line
     250-234 Text beginning with numbers
     250 The last line
  

Command-Reply Sequences

• See RFC5321

Size and Timeouts

• See RFC5321

Locating Servers

• DNS MX record
• or A/AAAA record

SMTP Authentication

• SMTP AUTH (PLAIN)
• C->S AUTH PLAIN (base64 encoded auth info)
• S->C 235 2.7.0 Authentication successful
• use STARTTLS (TLS)

Outbound Port 25 blocking

• ISP users can send Spam directly to an outer SMTP server
• Prevent this by blocking TCP port 25
• Mobile users can use mail submission agent (port 587) with authentication

Quiz of the day

• What is the number of TCP port in following message agents
• Message Transfer Agent
• Message Submission Agent

Homework

• Use "telnet is01 25" to connect our mail server and send yourself an e-mail message.
• Record the transaction and view the source of the message you've received.
• Include both the transaction and the message source in your report.