I have an interesting issue. I have a client that just got blacklisted due to spam being sent out over the last 2 days. I have my firewall configured to only allow mail to go outbound on port 25 from our mail server (Exchange 2010) exclusively and I have verified that there are no open relay's on our transport rules. We are running Vipre Business and after running deep scans with updated definitions all computers come back clean. I ran a message tracking report on our Exchange server that shows all mail sent via the mail server over the last couple of weeks and didn't see anything malicious or out of the ordinary. I have also verified that there are no home devices or rouge computers on the network. For all practical purposes it appears that the network is clean, but we still wound up on 5 or 6 blacklists...Where should I start looking next? Is there a "best practices" guide that can help eradicate this issue?

Thanks in advance! msindle