It looks like some large ISP's are blocking any mail from mail servers using a system like ASK. I got the following message from Earthlink and Comcast:
reason: 550 5.0.0 Stop sending undeliverable Spam Killer messages through our server)
Given how much spam originates from compromised Comcast users, it is somewhat ironic.
Has anybody else seen this and do you know where it originates from?
Randolf C. Richardson
> ... some large ISP's are blocking any mail from mail servers using a system like ASK ...
> ... do you know where it originates from?
The problem with C/R Systems  is that challenge eMail messages can be sent to trivially forged sender eMail addresses. As long as C/R Systems do this, they will get blacklisted because they are considered by most competent mail server administrators to be "fighting abuse with abuse." C/R Systems are also sometimes viewed as "spam amplifiers."
From a C/R System user's perspective, the "elimination of spam"  is very helpful, but unfortunately it's done at the cost of innocent bystanders who are victims of eMail address forgery.
From the perspective of other internet users, such as myself, each challenge that I receive but didn't cause to initiate is viewed as junk eMail (spam). Since I report all junk eMail to blacklists such as SpamCop.Net, because I believe in protecting my right to consent, there are occasions where C/R System challenges get reported too.
I hope that enough C/R Systems blockages occur that they'll stop being used, as the elimination of each form of internet abuse will get the internet back to a sane, usable state.
 Challenge/Response System.
 When you delegate your consent to the sender, as C/R Systems allow you to do in an automated fashion, even eMail that you would normally otherwise consider to be spam can no longer be labelled as such since it was sent to you with your consent.