This is probably a sendmail issue, not procmail. Although I do recall
past discussions of running outgoing mail through procmail, which would
make this possible, it's not as simple as it would seem. I don't know
how organizations get this done, but I doubt it's with procmail or any
"standard" sendmail configration. See:
http://www.sendmail.org/faq/section3.html#3.35
The issue pops up on postfix-users every now and so often. It is not
at all trivial. See http://msgs.SecurePoint.com/postfix/ for the
"adding lines to outgoing mail" thread (thread no. 19).
I believe the organisations that do it simply accept that they will
break a number of messages. Or they don't care.
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