For what it's worth, Netscape is one of those MUAs that can't cope with
"Content-Transfer-Encoding: binary". Our MIME parser is line-oriented,
and treats unencoded CR, LF, and CRLF synonymously. It would be a lot
of work to make it able to handle binary MIME parts.
This applies to both MIME and S/MIME objects; if an encrypted or
opaque-signed S/MIME message has unencoded binary data inside it,
that message won't be decoded properly.
So, I'd lean toward prohibiting cte-binary inside of S/MIME objects,
because it's less work for us (and I suspect for lots of other people
Jamie Zawinski http://people.netscape.com/jwz/ about:jwz