Alex van den Bogaerdt wrote:
Or you fail it with a 5xx SMTP failure code.
[Scenarios in which a 5xx can create backscatter]
Yes, I'm perfectly aware that 5xx's can cause backscatter.
But that's not my problem. It's up to all MTAs to avoid backscatter
and those that cause it will (rightly) get blacklisted until they
mend their ways.
A 5xx is the best way to reject unwanted mail because it doesn't
silently discard (which decreases confidence in e-mail) and it doesn't
compose an NDN (which causes backscatter). I realize that a 5xx is
not always possible and that you sometimes have to silently discard.
Rewriting the envelope sender to detect legitimate bounces vs. backscatter
can be done without an ESMTP extension.
Yeah, except that you seem to be looking at this from D's perspective.
If "D" is me, then yes: I'm looking at it from my perspective because
my mail server is the only one I can control!
I'm not. The key factor here is that the return path has to be at the
originator's ISP server. Or so I think.
If this is an issue, it can be solved (or ameliorated) with SPF.