[Exim] A lot of frozen mail problem

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Hi everybody,

Since two days, somebody on Internet, send a lot of mail (every 2 minutes) to an unavailable mailbox (@hotmail.com or @yahoo.com) using as FROM an unavailable address (always different set of random letters as local-part) from our domain. Therefore, the mailserver of hotmail or yahoo send a error mail for unavailable recipient to our mailserver that try to deliver it but the recipient doesn't exist too. Then the mail is frozen because the from address in the envelope of the mail sent bye the mail-daemon of yahoo or hotmail is equal to '<>' !

How is it possible to prevent this kind of mail bombing ?

PS : I hope that my explanation is not too confused ?

Raphaël
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