I´ve been hunting for a solution for SENDMAIL to avoid backscattering for quite a while.
See : http://spamlinks.net/prevent-secure-backscatter.htm doesn´t provide a working
example.

Since I solved it, I would like to share this with your guys and hopefully it will end up in
the SC FAQ http://www.spamcop.net/fom-serve/cache/329.html for sendmail,
so people won´t waste time...

Solution posted at:
http://elqui.dcsc.utfsm.cl/util/email/backscatter.html

best regards,