501 Connection rejected by policy [7.7]

Hello,

I have an email system on IP: 188.165.166.131

My users and I have been getting some mails returning with: "501 Connection rejected by policy [7.7]"

The server is not black listed, implements DKIM, and on http://ipremoval.sms.symantec.com/lookup/ it reports:

"The IP address you submitted, 188.165.166.131, does not have a negative reputation and therefore cannot be submitted for investigation."

Could you please check why this address is getting blocked?

Thanks,