Email being blocked by ATT, SBCGLOBAL and Comcast

I recently set up an Exchange 2003 server.  Everything is working fine except I get a bounce back from ATT/SBC with the following:

The message cannot be delivered due to a configuration error on the server. Please contact your Administrator.
            <server.domain.local #5.3.0 smtp;553 5.3.0 flpi128 - n4QKkdMD002152, DNSBL:ATTRBL 521< ###.###.###.###>_is_blocked.__For_information_see_http://att.net/blocks>

I have checked and know that I am not on any black list.  I think it is a setting in my SMTP Virtual Server Protocol.
In my Queue, there are about 5 domains that have email pending.  There is an error at the bottom that says "an SMTP protocol error occured".

Any ideas what I should look at to fix?
markcondiffAsked:
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Jerrod_WCommented:
Check to see if you're server is an open relay.
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Andres PeralesCommented:
did you click on their link and enter the information they requested...to find out what is going on!
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markcondiffAuthor Commented:
Yes, I followed their link and am not on any black list.
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markcondiffAuthor Commented:
Sorry to be nieve, but how/where do I check to see if it is an open relay?
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Andres PeralesCommented:
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Britt ThompsonSr. Systems EngineerCommented:
You'll need to verify that your mail server has an existing PTR record with your ISP.

mail.yourserver.com needs to respond to an arp query before you can send to those domains.
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markcondiffAuthor Commented:
I checked in the relay section and the radio button is selected that says "Only the list below": 127.0.0.1 and the exchange server IP are listed.
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markcondiffAuthor Commented:
Also, the check box is selected that says "all servers that successfully authenticate regardless of the list above".
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Britt ThompsonSr. Systems EngineerCommented:
Also, you'll need to go to your SMTP connector (not default SMTP virtual server) and go to the Address Space tab and make sure that "allow messages to be relayed to these domains" is unchecked.
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MesthaCommented:
AT&T run their own blacklists.
If you are not listed on the public list then you will need to call AT&T ask them why they are blocking your messages. It is an internal blacklist and only AT&T can remove you from it.

Simon.
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markcondiffAuthor Commented:
It is unchecked.  I have a feeling it is the lack of a PTR record.  I have seen this before with AOL.  I will add a PTR record and see if that helps.
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