Exchange 2003: 451 Could not complete sender verify callout

I am trying to track down meaningful info on the following error and MS Exchange 2003 server.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
The following recipient(s) could not be reached:

      Cathleen (sales@domainredacted.com) on 3/17/2007 2:13 PM
            The e-mail system was unable to deliver the message, but did not report a specific reason.  Check the address and try again.  If it still fails, contact your system administrator.
            <smtp.mydomain.com #4.0.0 smtp;451 Could not complete sender verify callout>
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

It's only happening when the user sends to one particular domain.  What I've found so far leads me to believe that the recipient's server is trying to probe my server to verify the originating email account, and my server is not responding.  Some other sites suggested that it might be my server's problem.  

Can anyone help?
maamisAsked:
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SembeeCommented:
The recipient is doing some kind of lookup.

Put your domain in to dnsreport.com and ensure that everything is correct in there.
If you have any firewalls etc, ensure that they are not scanning SMTP traffic.

Do you have recipient filtering enabled? If not, turn it and the tar pit on. However don't set the tar pit for longer than 5 seconds.

Simon.
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maamisAuthor Commented:
Did the DNSReport thing.  Everything is good in there.

I don't think I have recipient filtering on, and I don't know what the "tar pit" is...  Can you `splain that a liitle?  Thanks!
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SembeeCommented:
http://www.amset.info/exchange/filter-unknown.asp

It basically drops connections for users who are unknown.

Simon.
0
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maamisAuthor Commented:
Ok.  In the Message Delivery properties, "Filter recipients who are not in the directory" is checked.  I then went to the SMTP virtual server properties and checked the "Apply Recipient Filter" box.

Next, I did the tarpit.reg as outlined in your link.

But I'm not sure how this is going to help with mail we are sending out...
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SembeeCommented:
The server is calling back to your server. It should now get something positive back immediately.

This isn't strictly an Exchange issue, it is standard SMTP things. It is a matter of trying to work out what the remote site is looking for.

Do you have anything else scanning SMTP traffic? AV, Anti spam, firewall, appliance etc? Those can trigger that message as well.

Simon.
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maamisAuthor Commented:
We have GFI Mail Essentials for spam, but if I understand correctly, it's not looking at the port 25 traffic, it's looking at emails in the store.

Thanks for the help on this.  The user is trying her message again.
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maamisAuthor Commented:
The problem still occurs.
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SembeeCommented:
You will have to contact the other side and ask them what they are looking for.

Simon.
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