[lengthy explanation]
> I imagine the problem is in the sendmail config, something like this:
>
> Mlocal, P=/usr/bin/rmail, F=lsDFMAw5:/|@m, S=10/30, R=20/40,
> T=DNS/RFC822/X-Unix,
> A=rmail -d $u
>
> This is my particular setting for the "local" MDA, in my sendmail.cf.
> If I remember my sendmail configuration rightly, it is the "F" option in
> the "F=" section which determines that a "From " header will be added to
> the message before it is piped into the MDA. Now, for your case, you
> probably have P=/usr/bin/procmail, and since procmail creates its own
> message separator, having sendmail add it is superfluous; in that case,
> removing the "F" might be the way to handle this.
I agree with and understand everything you wrote above, but according
my copy of the bat book (1st ed only), the F flag in the F equate
means "Need From: in header", i.e. concerns From: and Resent-From:
headers, not the "From " mbox separator.
> At any rate, since this can be fixed at your site, perhaps it is best to
> do that, rather than coerce Mutt to adapt to your broken mail system.
Yep.
