Following a couple of mails with Mark Hennesy, I received the
following reply:

"It would appear that that mail challenge has already been responded
to, but we don't see that e-mail address in anyone's whitelist. If
another one gets generated, please e-mail to us the header for it
before answering it and we will get to the bottom of this."

I guess that each person only receives this once, hence I shouldn't
receive any further challenge. I would recommend the next person to be
bitten by this to contact staff <at> cloud9.net Either it is now
solved and no-one will receive another such challenge, or the next
person will have the chance to stop further senders to this list to
become "victims". Hopefully, only a very few of us managed to become
annoyed.

Set