### Known Mail/MIME fields... these, plus some general forms like
### "x-*", are recognized by build():
my %KnownField = map { $_ => 1 }
  qw(
  bcc         cc          comments      date          encrypted
  from        keywords    message-id    mime-version  organization
  received    references  reply-to      return-path   sender
  subject     to
 
  approved
);