E.2 The SMTP Envelope

The message envelope only consists of two pieces of information: the sender's e-mail address, and the recipient's e-mail address. When an SMTP client sends a message to an SMTP server, the SMTP server automatically creates the envelope for the message using the information provided in the "MAIL FROM:" and "RCPT TO:" commands. The SMTP server then hands the envelope and the message to a local delivery agent who will examine the envelope, and then place the message in the appropriate user's mailbox.

SMTP does not deliver the mail to the users mailbox directly. Instead it relies on a delivery agent to do this work. The delivery agent may be a Microsoft Mail or cc:Mail gateway for SMTP.

Figure E.1 below illustrates a simple SMTP connection:

Figure E.1 Simple SMTP mail session.

The "MAIL FROM:" and "RCPT TO:" information is captured by the SMTP server during the SMTP session, and are stored along with the message body. The local delivery agent examines the envelope to see who the message is for, and either places it in the destination user's mailbox or relays it on to the destination system.

For more information on the headers, refer to section E.3 Message Headers. For more information on the headers, refer to section E.3 Message Headers.

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