Return-Path missing via sendmail

[David Banning]

I am getting error on some of my mail processing because there
is no Return-Path line in received emails. I see that the line
H?P?Return-Path: <$g> -does- exist in my sendmail.cf.

Since sendmail is recieving email from the outside world
before delivering it to procmail it should be possible to 
get sendmail to put the line in there. I understand that
the Return-Path line will be inserted with the line 
"sendmail -f recipent at whatever.com", however I don't know where
sendmail processing is executed to add the -f option - and here I
don't even know if this is the answer - just picking up from others
posts.