501 Connection rejected by policy [7.7] (IP Blacklist Removal)

When we try to deliver messages to some domains we receive messages like "host cluster8.eu.messagelabs.com[85.158.139.35] refused to talk to me: 501 Connection rejected by policy [7.7] 17903, please visit www.messagelabs.com/support for more details about this error message."

The IP is not listed in any black list. We host a social media service (www.chainels.com) on two distinct but equal servers. One of them seems to be blacklisted (178.79.131.21), the other one is not (178.79.138.164).

Could you please help us to remove our IP 178.79.131.21 from your blacklist. We are getting reports from clients that they are not receiving our mails.

Thank you in advance.