Alex van den Bogaerdt wrote:
The big difference seems to be what happens after you decide a
message to be spam.
Present day: you delete it without notification or you send it "back",
Or you fail it with a 5xx SMTP failure code.
With TBR: you send a notification to the sender's domain. That server
should NOT forward the bounce to an innocent victim. Instead, the domain
could even count such bounces and be on guard about this customer.
Rewriting the envelope sender to detect legitimate bounces vs. backscatter
can be done without an ESMTP extension.