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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 Message Formats and Message Processing: RFC 822 and MIME                           9  TCP/IP Electronic Mail Standard Message Format: RFC 822

TCP/IP Electronic Mail RFC 822 Standard Message Format Header Field Definitions and Groups 1 2 TCP/IP Enhanced Electronic Mail Message Format: Multipurpose Internet Mail Extensions (MIME)

The standards that define SMTP describe the protocol as being responsible for transporting mail objects. A mail object is described as consisting of two components: a message and an envelope. The envelope contains all the information necessary to accomplish transport of the message; the message is everything in the e-mail message we have seen in the last two topics, including both message header and body.

The distinction between these is important technically. Just as the postal service only looks at the envelope and not its contents in determining what to do with a letter—no wise-cracks, please! J—SMTP likewise only looks at the envelope in deciding how to send a message. It does not rely on the information in the actual message itself for basic transport purposes.

So technically, the envelope is not the same as the message headers. However, as you can tell by looking at the list of e-mail headers, each message includes the recipients and other information needed for mail transport anyway. For this reason, it is typical for an e-mail message to be specified with sufficient header information that it can be considered enough by itself to accomplish its own delivery. E-mail software can process and interpret the message to construct the necessary “envelope” for SMTP to transport the message to its destination mailbox(es). The distinction between an e-mail message and its envelope is discussed in more detail in the topic describing SMTP mail transfers.

TCP/IP Electronic Mail RFC 822 Standard Message Format Header Field Definitions and Groups 1 2 TCP/IP Enhanced Electronic Mail Message Format: Multipurpose Internet Mail Extensions (MIME)