message envelope

The information contained in an e-mail message that is required to send and deliver a message between messaging servers by using SMTP protocol commands. Defined in RFC 2821, the message envelope contains the sender e-mail address and the recipient e-mail address information used in MAIL FROM: and RCPT TO: as the message is routed from server to server for delivery. Recipients never see the message envelope, because it is not part of the message content, and it is discarded when the message reaches its destination. The X.400 messaging standard equivalent to the message envelope is called the "P1 header." The message envelope is often referred to as the "P1 header." [Exchange 2007]