Figure 1-2 shows this in a greatly simplified conversation between the local
sendmail and the remote machine??™s sendmail.
The local sendmail tells the remote machine??™s sendmail that there is mail from you
(the envelope-sender) and for friend2@remote. It conveys this envelope-sender and
recipient information separate from and before it transmits the mail message that
contains the header. Because this information is conveyed separately from the message
header, it is called the envelope.
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10 | Chapter 1: Some Basics
Only one recipient is listed in the envelope, whereas two were listed in the message
header:
To: friend1, friend2@remote
The remote machine should not need to know about the local user, friend1, so that
bit of recipient information is excluded from the envelope.
A given mail message can be sent by using many different envelopes (like the two
here), but the header will be common to them all. Note that the headers of a message
don??™t necessarily reflect the actual envelope. You witness such mismatches
whenever you receive a message from a mailing list or receive a spam message.
1.6 Basic Roles of sendmail
The sendmail program plays a variety of roles, all critical to the proper flow of electronic
mail.
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