Why I am sceptic about allowing forwarding formerly encrypted mails
unencryptedly or after re-encryption is that - for me - forwarding shouldn't
change the original message.
I agree with this much. A UA that claims to be forwarding a message shouldn't
misrepresent the message's content. If the subject message is encrypted, the
corresponding content of the forwarded message should also be encrypted.
It would be fair for the UA to refuse to forward such a message to someone
who wasn't a recipient - or at least to warn the forwarder that the recipient
probably won't be able to read it.
OTOH if only part of the message is forwarded, say the body or an attachment,
the original encryption has to be removed anyway.
It seems like the issue here is really one of how the forwarded message is
represented to the ultimate recipient - does it look just like any other
forwarded message, or can the recipient tell that it's been altered?
Have we ever defined exactly what a forwarded message should look like anyway?