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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 Connection and Session Establishment and Termination 1 2 3 SMTP Special Features, Capabilities and Extensions

The Rationale for A Separate E-Mail Message and Envelope

In fact, one question that sometimes comes up when examining SMTP is why couldn't this process be even simpler? The first two steps identify the sender of the e-mail and the intended recipient(s). But all of this information is already contained in headers in the message itself. Why doesn't SMTP just read that information from the message, which would in fact make the mail transaction a one-step process?

The explanation for this isn't specifically addressed in the SMTP standards, but I believe there are several reasons:

• Specifying the sender and recipients separately is more efficient, as it gives the SMTP receiver the information it needs “up front” before the message itself is transmitted. In fact, the SMTP receiver can decide whether or not to accept the message based on the source and destination e-mail addresses.
  
  
• Having this information specified separately gives greater control on how e-mail is distributed. For example, an e-mail message may be addressed to two recipients, but they may be on totally different systems; the SMTP sender might wish to deliver the mail using two separate SMTP sessions to two different SMTP receivers.
  
  
• In a similar vein, there is the matter of delivering blind carbon copies. Someone who is “BCC'ed” on a message must receive it without being mentioned in the message itself.
  
  
• Having this information separate makes implementing security on SMTP much easier.

For these reasons, SMTP draws a distinction between the message itself, which it calls the content, and the sender and recipient identification, which it calls the envelope. This is of course consistent with our running analogy between regular mail and e-mail. Just as the postal service delivers a piece of mail using only the information written on the envelope, SMTP delivers e-mail using the envelope information and not the content of the message. It's not quite the case that the SMTP server doesn't look at the message itself, just that this is not the information it uses to manage delivery.

SMTP Connection and Session Establishment and Termination 1 2 3 SMTP Special Features, Capabilities and Extensions