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To prevent Backscatter, it was suggested to bounce only to local recipient.

How to do this in KMS?

Thanks in advance.


Preventing Backscatter

If you run a mail server you have a responsibility not to send backscatter. Bounces should ideally only be generated by a mail server to a local recipient. Mail servers should not generate bounces to non-local recipients, but should instead reject the mail during the SMTP session, and leave the remote sending server to handle the bounce: if a rejected mail is a legitimate message, the bounce gets generated by the remote sending machine, as expected; if a rejected mail is not a legitimate message, the remote end will probably not generate a bounce, and all is still well.

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Unless you have a * (catch all) alias and reject email somewhere else, I think KMS is not generating any unwarranted backscatter. Unknown recipients are cut-off during SMTP, spam is silently dropped. Out of office messages are legitimate bounces. What else?

Do you have examples when KMS is sending bounce emails when it in your opinion shouldn't?

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Backscatter generally occurs in two scenarios.

The first is you have anti-virus or spam/content filters set to send a notification to senders that their mail didn't pass. This is almost always a bad idea.

The second occurs when you accept mail, then reject it for some reason. This typically happens with an SMTP gateway or a backup MX server. If that host doesn't have any knowledge of your valid address list, it accepts everything for the domain, then forwards it on to the next server. If the address is no good, or the mail gets refused for any other reason, that host then has to send a non-delivery receipt. If that host gets pumped full of mail from a spammer or virus, this can generate huge amounts of backscatter.

There a few places I see in KMS that could/would generate backscatter. Under Configuration -> Content Filter -> Spam Filter:
On the Spam Rating tab, "Send bounce message to sender".
On the Custom Rules tab, "Send bounce message to sender".

Under Configuration -> Content Filter -> Attachment Filter:
"Send the sender a warning informing that the attachment was not delivered".

As long as those are off, you shouldn't generate backscatter.

If you have an SMTP gateway or backup MX server, it will depend on how that is set up whether it generates backscatter or not.

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