Each mail message must have a sender.
The sendmail program can determine the sender in
four ways:
If the sender is specified in the envelope of an SMTP connection,
that sender's address is used.
If the -f command-line argument is used to run
sendmail, the sender's address
is the address following the -f.
If the sender is not specified in the envelope, the address that is
used is that of the user who ran the sendmail
program. If that user is unknown, the sender is made to be
postmaster.
When processing the queue, the sender's address is
specified in the S line of the
qf file.
The form of the S line in the
qf file looks like this:
Saddr
The S must begin the line. Exactly one address
must follow on that same line. Whitespace can surround that address.
There can be only one S line in the
qf file.
If the addr is missing,
sendmail sets the sender to be the user who ran
sendmail. If that user is not known in the
passwd file (or database),
sendmail syslog(3)s the
following message and sets the sender to be
postmaster:
Who are you?
The resulting address is then processed to extract the
user's full name into $x ($x). Finally, the sender's
address is rewritten by the canonify rule set 3,
the parse rule set 0, and the
final rule set 4.
Under all versions of sendmail the address in
the S line will include any RFC822 comment text
that appeared with the original message. Under V8.7, if the
F=c flag (F=c) is set for
the sender's delivery agent, all comment text is
stripped from the address.
If sendmail is compiled with USERDB defined
(USERDB), the sender address can optionally be
rewritten by the User Database before it is placed into the
S line. Such rewriting is allowed only if the
delivery agent for the sender includes the F=i
flag (F=i).
