One individual on this list has set up blocking of messages from this list
unless they receive confirmation. I am willing to send a one time
confirmation, but not to an individual person every time I post. I am even
willing to confirm messages once every time I post if the listserv is setup
that way.
There is a point where the security becomes onerous, and this is definitely
it. The prospect of confirming each message to each individual on a list or
on a private mailing list is not reasonable. Now if there were a standard
way of having confirmations generated automatically, that would be a
reasonable way of confirming that a message did indeed come from the claimed
sender. Of course the sender would have to have an intelligent mail program
that checked the reply request against sent messages.
A mechanism of this kind would be intriguing, as it could potentially put an
end to most spam, but it will not work until everyone uses it.