Exchange Public IP is getting Blacklisted again and again at CBL

Hi,

My Exchange server Public IP address is getting Blacklisted at CBL again again ( 2-3 times in last one month). It's a new server we setup 2 months ago. I found this blacklisting this morning also and get that cleared in 2 hours. But still  my couple of users getting non-deliveralble from few emails receipients with error msg like below:

"ESMTP rejected your message to the following e-mail addresses:
 
ESMTP gave this error:
Transaction Failed Spam Message not queued.
 
A problem occurred during the delivery of this message to this e-mail address. Try sending this message again. If the problem continues, please contact your helpdesk."

My questions are:

- do you think this is because of an Internal PC infection or a server configuration issue
- Should I ask my ISP change my Static IPs - do you think if that will help?
- any other suggestions? (my inbound email comes through postini just in case if you need this info)


Any help/advice  will be highly appreciated.
Byas_SahaAsked:
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PapertripCommented:
- do you think this is because of an Internal PC infection or a server configuration issue
Wouldn't be surprising at all if it is an infected internal PC.
- Should I ask my ISP change my Static IPs - do you think if that will help?
No.  Unless you fix the source of the problem your new IP will just get listed like the old one.
- any other suggestions? (my inbound email comes through postini just in case if you need this info)
Block all traffic from internal -> internet:25 except for your outgoing mail servers.  If problem persists, you will now have logs of it to help find the culprit(s).
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