Trouble with Anti-spam Content Filtering

I have an E2K7 server that has been online for a few weeks now, running very smooth.  I have recently been getting reports, however, from some senders whose messages are not being received.  Here is the message that one sender has received, which is my main concern:
"Remote host said: 550 5.7.1 Message rejected as spam by Content Filtering."

This particular sender is able to successfully send messages to one of my users, but gets the above message when sending to two of my other users.  The tools I am using are the Anti-spam agents that come with Exchange Server 2007, and the only one I have enabled is the "Content Filter".  The settings for my filter are as follows:

Reject messages with SCL rating greater than or equal to 6
Quarantine messages with SCL rating greater than or equal to 4.

I closely monitor the Quarantine folder, but cannot remember seeing quarantined messages from this particular sender.  When I use the Mail Flow Troubleshooter, with the problem sender and recipient addresses, I do see an error that says the Client is not authenticated.  When I do this test, the test messages do appear in my spam Quarantine folder.

Does anyone know why this sender would be able to get through to one of my users, but not others?  Is there a way to manually enter email addresses that are authorized to send to us, given the filter setup that I am using?

Thanks.
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Rob GrinageManagerAsked:
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.Commented:
Hi, If you run this cmdlet:

Get-AgentLog -EndDate 04/11/2008 -StartDate 01/11/2008 > c:\filter.txt

it will create a file in the root of c: called filter.txt , if you search the file for the senders email address you will see additional infomation as to why it was blocked... e.g.

SmtpResponse    : 550 5.7.1 Recipient not authorized, your IP has been found on
                   a block list
Reason          : BlockListProvider
ReasonData      : spamcop
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