I can state with authority that mail with sender==recipient is not universally 100% spam, and such a policy would likely have a much higher false positive rate than zen. You can argue it's a misconfiguration of the sender, but a mail admin's job is to receive legit mail. but you're welcome to reject mail on any basis you see fit. a small site can probably implement this policy with no noticeable problems.
Sure. I'm sending myself emails sometime. But I'm using server which is permitted to send with address from my domain. So that's surely not 100% spam when sender eq recipient. But then we come to definition of spam. It's in simple words unwanted message. And when someone spoofs my email address, it's certainly not obeying with my image of legit mail. If I'm calling someone, I'm not presenting myself as John Johnson. Maybe some people do... I think you can't give any example that one ^have to^ use my email address when sending msg to me. You didn't convince me that I'm rejecting any legit mail by rejecting not authenticated users.
Also IMHO I'll get much more "false positives" with zen then with authentication if for example I'd be interested in getting money and medicines offers. We get here to definition of "false positives" which can be very different for different customers. And that leads as to another problem whether we consider authentication at customer level (or server serving SMTP for just single company) or site-wide (in terms of many customers with single configuration).
Coming back to large ISPs. After checking few "emails for everyone" providers in Poland I can tell that all of them do relay only for domains they serve. And it's not common for Internet Access Providers to block outgoing connections to port 25, still I mean here, in Poland. So I think that situations pointed by you are rather rare. I don't know of any, so I'm fine with rejecting 0 legit mails at my place by using smtp authentication.
Pawel Lesniak
