One of the subscribers to my free newsletter is using a service called Cashette for spam control. This service appears to require me to sign up in order to get my mail delivered to the individual. Like many of the challenge/response systems, it requires you to click a link in an email message. When you get to what should be a challenge/response phrase, Cashette asks for a username and password.
I don't have a Cashette account, so I click the link that says "Why should I signup?" The response:
"1) Cashette will cover your cost up to $0.25 to send email to clueless@mydomain.com."
"To stop spam, clueless@mydomain.com is charging a small fee to those who are not yet on his/her Approved List. The payment is automatically refunded when he/she replies to you or adds you to the Cashette address book or Approved List."
"2) You can stop spam for your existing email addresses. Learn more."
"3) You get a free email account with 10MB of storage space. Learn more."
So if I want to deliver the message to this subscriber, I either pay $0.25 or signup for Cashette. Guess he doesn't need the newsletter.