From: Victor Duchovni (Victor.Duchovnimorganstanley.com)
Date: Mon Nov 16 2009 - 14:10:48 CST

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On Mon, Nov 16, 2009 at 12:53:14PM -0700, Jim Lang wrote:

> OK here is the scenario.
> Spammer sends mail to: usermyclientsdomain.com from forged address
> victimrandomdomain.com
>
> If usermyclientsdomain.com is delivered locally, not a problem, if the
> address is invalid, postix rejects the mail during the smtp connection.
>
> But if usermyclientsdomain.com is an alias to myclientotherserver.com,
> postfix accepts the mail as deliverable and forwards it to hotmail.com.
> But if myclientotherserver.com can for whatever reason not be delivered,
> otherserver.com does what it is supposed to do and rejects the mail during
> the smtp connection, which causes postfix to send out a non-delivery
> report to victimrandomdomain.com -- backscatter.
>
> Is there a way to stop this?

Some backscatter is unavoidable, you can keep the volume low by removing
local aliases to no-longer-valid external addresses, and by rejecting
mail from spam sources, using good blacklists, ...

--
        Viktor.

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