Non-delivery is a serious failure of the mail system. It must not go unreported. Some would argue it should be delivered to both parties. It could make sense for the sender to decide who to deliver it to, though you can’t easily stop the recipient from superseding that.

I agree. That's what smtp has DSNs for. Any 550 error message (which turns into a mailer-daemon notification in the sender's mailbox) we issue has something like "Mail refused, please see http://spamblock.outblaze.com/202.54.30.2" (or whatever the blocked IP is).

Similarly for blocked domains, or for other filter rules. And that's accompanied by a fairly easy auto removal mechanism that can be used just once - and a link to contact the site postmaster (me and my staff) - and we respond to tickets reporting false positive blocks within a business day, or even sooner.

Blocking is going to keep happening, and C/R bots just dont scale .. but blocking should be done responsibly and should be blocked by people who are willing to listen and respond fast to false positive reports.

srs