the pgp_check_exit option (enabled by default) that causes mutt to
check the exit code of the PGP subprocess.  A non-zero exit code means
that the subprocess failed and mutt will not continue to send the
message.

This is needed as in certain cases, PGP or GnuPG can fail to
completely process a document (say, if the gpg.conf file is mangled,
which is how I discovered the problem).  Without an exit code check,
mutt will continue anyway and send the half processed file.