I have been thinking of maybe putting up an experimental anti-spam blocklist server. As far as the client interface, this would operate in the usual way, i.e. via DNS, just as all of the current well-known blacklists do.
Due to the (backend) nature of the thing however, it would probably only provide service ... initially at least... from a single machine and a single IP address. I am curious what would happen to Postfix clients if they included references to that in their smtpd_recipient_restrictions, and if the one and only server for the thing went down for a time. What would happen in such a case? Would inbound e-mail start to back up horribly, as Postfix waited for DNS responses that were not forthcoming?
