Most (all) MIME UAs will have some defaulting rules. For subtypes they
don't recognize the behaviour will default in ways dependant on the type.
So message/pem-clear will default to message/rfc822 (right?),
It shouldn't. Message subtypes exist which are not structurally identical
to message/rfc822, and the MIME specification gives no reason to assume
that future message subtypes will in fact all be compatible.
and the
UA should give the user access to any body parts enclosed within. Thus
the tagging will be ignored, but at least the user will still have access
inwards.
The rationale for using a different subtype of message is that it should
be easy to get UAs to do just this, whereas MTAs and gateways will not.
Ned