How to stop a Joe Job spammer??

One of my resellers customers has become the victim of a joe job and all the bounces are returning to my server filling the mail queue I am talking 20,000 bounced mails in a day.

The domain did not send the mail but instead the header has been spoofed with an address like sdkaiw42234@domain.com of course the first part changes with every email to random characters and because domain.com is hosted on my server all the bounces are returning to me.

Any ideas how to filter them or stop it?
Already tried blackhole but it does not seem to be working and besides the mail still comes in eating incoming bandwidth.

Short of terminating the account that is innocent, there has to be some way to stop this.

 

 

 

 

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