multipart/blah

Does anyone know of a specific MUA that doesn't properly deal with
multipart/blah?

By 'properly deal with' I am referring to MUAs which don't recognize
"blah" and also don't treat the multipart as if it were multipart/mixed
(per RFC2046, section 5.1.7).

I have tested Netscape V4.5's mailer and Eudora, both of which seem to 
be happy.

Thanks,
-Dan Wing