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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 Mail Transaction Process
(Page 2 of 3)
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
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.
Note: It is possible for the sender of a message to generate envelope information based on the contents of the message, but this is somewhat external to SMTP itself. It is described in the standard but caution is urged in exactly how this is implemented.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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