sendmail, 4th Edition

The Envelope

So that it can more easily handle delivery to diverse recipients, the sendmail program uses the concept of an envelope. This envelope is analogous to the physical envelopes that are used for post office mail. Imagine you want to send two copies of a document, one to your friend in the office next to yours and one to a friend across the country:

To: friend1, friend2@remote

After you photocopy the document, you stuff each copy into a separate envelope. You hand one envelope to a clerk, who carries it next door and hands it to friend1 in the next office. This is like delivery on your local machine. The clerk drops the other copy in the slot at the corner mailbox, and the post office forwards that envelope across the country to friend2@remote. This is like sendmail transporting a mail message to a remote machine.

To illustrate what an envelope is, consider one way in which sendmail might run /usr/lib/mail.local, a program that performs local delivery:

                       deliver to friend1's mailbox
                    ↓
/usr/lib/mail.local -d friend1         ← sendmail runs
                          ↑
                           the envelope recipient

Here sendmail runs /usr/lib/mail.local with a -d, which tells /usr/lib/mail.local to append the mail message to friend1’s mailbox.

Information that describes the sender or recipient, but is not part of the message header, is considered envelope information. The two might or might not contain the same information (a point we’ll gloss over for now). In the case of /usr/lib/mail.local, the email message shows two recipients in its ...