Question : I'm having email bouncing back undeliverable from one client machine

I'm having problems with one client machine sending emails. He has had 5 different messages bounce back undeliverable. See errors below. I have had this problem on many machines in the past until I set up reverse DNS with my ISP everything has been fine until now. I checked my IP (65.45.15.52) to see if its on any blacklists and found nothing. Can you give me any suggestions where to look next?
Thanks.
I found that my Exchange 2003 Server wasn't automatically updating it's anti-virus. I fixed that issue and have sent an email to one of the Hotmail accounts and haven't had it bounce back as of yet.


      'xxxxx  xxxx' on 2/7/2008 5:24 PM
            554 5.7.1 : Recipient address rejected: Access denied

      'xxxxx  xxxx' on 2/7/2008 5:24 PM
            554 5.7.1 : Recipient address rejected: Access denied

            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            http://postmaster.live.com for email delivery information and support>

            http://postmaster.live.com for email delivery information and support>


            You do not have permission to send to this recipient.  For assistance, contact your system administrator.
            ... Mail from 65.45.15.62 refused due to zombie blacklisting, sender is www.com>>

Answer : I'm having email bouncing back undeliverable from one client machine

maxis2cute, there is an MX record for dqs.com, that's enough. So everything is fine with DNS.

Read http://www.commtouch.com/documents/Commtouch_2006_Spam_Trends_Year_of_the_Zombies.pdf
for more info about Zombie spam senders.

Either your Exchange server or any of your client, that uses your Exchange had been infected by some kind of Trojan horse virus, that made it a part of spam bot network.

Remote mail servers with antivirus software detects such 'zombie', because they start to send massive infected mail and adds IP to blacklist.

So I would recommend you to:
1) Check all your clients and servers for viruses with fresh AV database
2) AFTER you ensure that no spam bot is installed on ANY of your mail clients, you should follow recommended actions (say following http://postmaster.live.com or writing to to comcast.net support) and ask site mail admins to exclude your host from their blacklist.

You may also analyze your own mail logs to find out massive mail transfers from a single IP, that IP is most probably infected.