Having been a
SpamCop user for two years, I
strongly agree with (b). I don't think any of Infopop's products comply with the confirmed opt-in provisions of
MAPS Basic Mailing List Management Principles for Preventing Abuse. Although UBB.classic, UBB.threads and OpenTopic are not mailing lists, they have mailing-list-like features, and should conform to those guidelines.
(a) would be useful, but not sufficient without (b). Almost all of the spam I get has an opt-out link.
Regarding (c), bounce checks would also be useful. I'd prefer that after the three bounces, the user gets flagged (U_Bounce or something), and an admin could choose whether or not to remove the user or try to resolve the issue.
