P.S.  Whoever is curious on what I'm talking about can try a session
like below, and see the From: field of the message delivered to you.
(The same phenomenon happens to To: and Cc: fields, of course, but you
cannot test it without bothering the real server at caml.inria.fr.:-)

The result is "From: nosuchuser@brion.inria.fr" in all sendmail
                                ^^^^^
installations that I have access to.  One can also try:

> grep DontExpandCnames /etc/mail/sendmail.cf
O DontExpandCnames=False
> 

	Eijiro

----------------------------------------------------------------------
> telnet YOUR.SMTP.SERVER smtp
Trying aaa.bbb.ccc.xxx...
Connected to your.smtp.server (aaa.bbb.ccc.xxx).
Escape character is '^]'.
220 your.smtp.server ESMTP
helo YOUR.HOST.NAME
250 your.smtp.server Hello your.host.name [aaa.bbb.ccc.yyy], pleased to meet you
mail from: nosuchuser@caml.inria.fr
250 2.1.0 nosuchuser@caml.inria.fr... Sender ok
rcpt to: YOUR_USER_NAME@YOUR_DOMAIN_NAME
250 2.1.5 your_user_name@your_domain_name... Recipient ok
data
354 Enter mail, end with "." on a line by itself
test
.
250 2.0.0 j2VJmv1a026374 Message accepted for delivery
quit
221 2.0.0 your.smtp.server closing connection
Connection closed by foreign host.