Authentication warning on sender ... yuck

[Harry Putnam <reader@newsguy.com>]

Once more raising the question of what gnus does concerning a "Sender"
line and where gnus stops and sendmail (mta)  starts.

I've been noticing this line in my messages:
Authentication-Warning: reader.ptw.com: reader set sender to blah blah 


Far as I know I have not done that.  I am masquerading ptw's mail
machine though.

It seems to be coming directly from gnus... Here is why I say that:

Tailing my maillog as I send test messages from gnus, I see that
warning messages on out-going messages.  

Using berkely mail,  I see the outgoing message, it contains no warning
Using mutt, I see the outgoing message, it contains no warning.

All those mail clients use sendmail as mta, but only messages sent
with gnus contain that warning.  I do have this in .gnus.

(setq message-syntax-checks '((sender . disabled)))