What if, in a mail user agent, the user wants to forward an encrypted message?
Allow it? Deny it?
Re-encrypt or remove encryption?
The problem is, of course, that the original sender might not like his
encrypted text being sent out in the clear again...
I don't think this is something we can control. If you encrypt something with
a recipient's public key, you're implicitly giving the recipient the ability
to decrypt that message and redistribute it to his heart's content. You can
issue instructions to the recipient about redistribution of the messgae, but
you can't control whether the recipient follows those instructions.
Nor is it clear that this is a "problem". At least, there appear to be
more "problems" associated with mechanisms that do purport to control what
a recipient can do with a message than mechanisms that merely provide
protection against interception of a message in transit from sender to
recipient.
Keith