> and then spammed me to boot

There is a misunderstanding about how this works and how email works:

* Rejecting an email during the SMTP exchange results in the /sending/ server returning a response to the sender. This is NOT spam and it not under the control of the receiver.

* Rejecting an email post-acceptance and sending a reply is called /blowback/ and is typically considered poor practice and a type of spam. (But not UCE.)

* Adding a header to an outgoing message which causes the recipient's email server to possibly generate an email message to the recipient when they reply has nothing to do with the sender. The recipient's email provider is doing this, and the problem is you.

Let's recapitulate:

1) You use Microsoft as an ESP.

2) Somebody sends you a message, received by your ESP with this header.

3) Your ESP takes an action based on this header which results in you receiving feedback.

The whole thing is a shitshow, oblivious to externalities; but whatever.

Nobody will spam you except your service provider. My mail server will just return error to your service provider when it tries to send email to it over SMTP.

I agree, this is clearly worse, whether it's "your e-mail has been rejected" or "this server is not configured to accept 'reactions'", it's unnecessarily hostile to a user who probably doesn't understand that Microsoft has done something non-standard. And they will probably assume that you don't know what you're doing, because your e-mail doesn't work.

The reaction sender's Exchange server would see it and could decide to not display the rejection to the user, since the Exchange server knows it's "just" a reaction that was rejected.