Wow; I haven’t gotten one of these in a long time:
A message you recently sent to a 0Spam.com user with the subject "[redacted]" was not delivered because they are using the 0Spam.com anti-spam service. Please click the link below to confirm that this is not spam. When you confirm, this message and all future messages you send will automatically be accepted.
I wrote about challenge/response anti-spam systems about three years ago, but probably haven’t seen a challenge message in at least two years. I thought people had given up on them.
Alas, no. But if the last two years is something to judge by, they’ve at least fallen further into disfavour.
Anyway, it’s worth a re-post, then, of my three-year-old item about them. All the problems, all the reasons one shouldn’t use them, are still valid now. So, here’s the link again: head over and read (or re-read) it.