Exchange 5.5 & Spam

[rghz]

Hello

I have been receiving some Inbound/Outbound messages from our exchange server for users we have not tried to send emails to with bad site advertising. We receive errors if the target email account does not exist, so it seems it is attempting to send out the emails.

We have relaying prohibited and I have verified that the relaying is prohibited and working but I still get these failures. Why?

Here is an example of a Inbound faiulure:
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The following recipients did not receive the attached mail. Reasons are listed with each recipient:

<Kara_bq842@yahoo.com> Kara_bq842@yahoo.com
MSEXCH:IMS:CSA Waverley:Wooburn Green:EXCHANGE 3554 (000B09AA) 554 delivery error: dd This user doesn't have a yahoo.com account (kara_bq842@yahoo.com) - mta607.mail.yahoo.com
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Any help would be very much appreciated.

 

[Griffyn]

The reason why this is happening is because the spam sender's e-mail address is not valid. It is completely possible to send mail with no sending e-mail address, or a bogus one.

So what is happening is that such an e-mail is arriving at your domain, but with an invalid user portion. ie. xxxx@yourdomain.com

Your exchange server sees that no user matches, and so tries to send an NDR. The NDR is sent out, but then the yahoo mail daemon replies with it's own NDR. It is this final message that you're seeing.