D G Teed wrote:
[snip]
I'm afraid this is misunderstood, or I didn't explain it carefully enough.
The ISP sending the bounce notification is my home ISP, not
the ISP for my work. At home I run a small postfix
which relays all outbound to my home's Cable ISP's SMTP.
The Cable ISP's SMTP attempts delivery to one of
the MX servers at work. The user doesn't exist, so the
Cable ISP must send a NDR to the sender - my home
email account.
If my email client at home used the Cable ISP's SMTP
then I could see how it would reject rather than bounce,
but because there is a relay early in the hops, that does not
happen.
I'm sure spammers can make the same thing happen.
Then it's the home ISP problem. How the ISP deals with this problem may
vary. some ISPs do nothing. others will discard undeliverable mail if
the original sender isn't in their domain(s) (some extend this to a list
of domains that the subscriber must declare), ... etc.
Note that this problem is different from the "general" backscatter
problem. Here, backscatter is caused by a "submitted" mail, not by mail
sent to an MX that fails to validate recipients. In short, only
subscriber machines can cause this backscatter.
[snip]
