A 501 return will cause an email bounce, so the sender will know that it has been refused. If you for example use "501 5.6.1 AMP format email messages not supported, please use a mail system that doesn't use AMP" then the sender will get that as the reason the mail was not sent.

I know, but AMP will be used mostly for automated email, and I'm not holding my breath many services that send me e-mail check their mailserver logs, if they even know such a thing exists. Bouncing to no-reply address will not help much either. And I need to receive e-mail from services I've registered to anyway, that's why I whitelist anything sent to a random email address that I assign to each service.

Better to just complain over channels where someone is actually listening.

>Bouncing to no-reply address will not help much either.

If someone does that, it's a good method to land on basically any spam blacklist out there.

Most larger providers like mailgun will tell you if you're bouncing a lot of mails and reduce your reputation. Same for gmail even.

Hmm, so if more people do this, anti-amp email blocking would reduce reputation of amp-email senders? Nice.

Indeed, atleast for most popular email providers that should be the case.

it also helps if you report senders that repeatedly bounce that way to spamlists. That usually gets a lively reaction.