501 Connection rejected by policy [7.7]

Hello,

Our organization has been recently recieveing bounceback e-mails with the following content:

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:

    host cluster1.us.messagelabs.com [216.82.249.179]

    SMTP error from remote mail server after initial connection:

    501 Connection rejected by policy [7.7] 4412, please visit www.messagelabs.com/support for more details about this error message.

Is it possible to ublock our IP adress: 198.49.76.18 

Thanks,

Wolken.