The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)

SMTP Security Issues 1 2 3 SMTP Replies and Reply Codes

Obsolete SMTP Commands

The commands in the preceding table are the ones that are most commonly used in SMTP today. In addition to those, there are also certain commands that were originally defined in RFC 821 but have since become obsolete. These include the following:

• SEND, SAML (“send and mail”) and SOML (“send or mail”): RFC 821 defined a distinct mechanism for delivering mail directly to a user's terminal as opposed to a mailbox, optionally in combination with conventional e-mail delivery. These were rarely implemented and obsoleted in RFC 2821.
  
  
• TURN: Reverses the role of the SMTP sender and receiver as described in the SMTP special features topic. This had a number of implementation and security issues and was removed from the standard in RFC 2821.

SMTP Security Issues 1 2 3 SMTP Replies and Reply Codes