So what's so bad about forwarding all tcp/25 traffic over that relay and letting that relay decide if the MAIL FROM: is allowed to be relayed?
Because I want to send mail through my own SMTP server that speaks STARTTLS and uses certificates that are under my control. Maybe I don't want my email sitting around in your MTA queue for your sysadmins to read. Or maybe you just don't have a clue about how to configure and run an MTA, therefore any mail I send through your enforced gateway gets silently black-holed.
And if a client wants to mail from another domain which isn't relayed by it's upstream ISP, he/she could ask it's ISP to do so. Yes this will add an administrative hassle, but doesn't spam imply that also?
Do you *honestly* believe what you wrote above? Do you have any experience trying to actually get these sort of changes made? Can you provide statistically valid numbers showing this is a realistic solution in the real world? (Frankly, this proposal is so absurd I have to wonder if you've even dealt with *an* ISP ...) The Internet is a peer-to-peer network, whether you like it or not. --lyndon Lizzie Borden took an axe, And plunged it deep into the VAX; Don't you envy people who Do all the things YOU want to do?