auto-accept friends of friends

Hi,

I'd like to auto-accept other recipients of mail from trusted friends.
For instance, when someone I trust sends a mail to a group of people
(with several recipients in both To: and Cc: fields) I'd like to
automatically accept mail from others that reply later. The fact is that
a lot of non-tech-savvy people find qconfirm messages confusing or even
offensive, and I'd like to keep challenges to a minimum to people who
are probably not sources of spam.

I realize there should be some differentiation between trusted and
non-trusted senders (otherwise it would be easy for a spammer to send a
mail to me with Cc: to 1000 addresses and confirm once and have access
to loads of addresses to spam from, whereas I could have trouble finding
them all in my accepted list), and there would have to be some concept
of addresses that are mine (don't want to confirm those), but I think it
could be useful.

Any suggestions? Any gaping holes in this idea that can't be closed?

Ketil