Outbound spam issue

After hours, the mail server is being hit with thousands of outbound spam messages coming from postmaster@domain.  Postmaster does not exist.  It's reaking havoc on our network, causing us to be blacklisted etc.  I have scanned every pc, checked for open relay, firewall is configured for all outbound smtp traffic to go thru mail server.  No matter what, mail keeps flooding network, usually after 5pm and on weekends.  
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szagoriaAsked:
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Alan HardistyConnect With a Mentor Co-OwnerCommented:
You are probably on the receiving end of an NDR attack.

Please have a read of my Article that should help you resolve this problem by enabling Recipient Filtering:

http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/A_2556-Why-are-my-outbound-queues-filling-up-with-mail-I-didn't-send.html
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