501 Connection rejected by policy [7.7] 6207

Hello,

We have emails from our server to your network failing with the below errors. Our IPs doesn't appear to be in any of the major spam database as well. Please review and provide a solution.

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This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

xxxx@xxxx.com

    host cluster1.ap.messagelabs.com [117.120.16.147]
    SMTP error from remote mail server after initial connection:
    501 Connection rejected by policy [7.7] 6207, please visit
www.messagelabs.com/support for more details about this error message.

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When we try to delist at http://ipremoval.sms.symantec.com/lookup/ , it says 

The IP address you submitted, x.x.x.x, does not have a negative reputation and therefore cannot be submitted for investigation.