501 Connection rejected by policy [7.7]

Hello,

I have an email system on IP: 93.158.211.31 and 93.158.211.34

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

The IP's mentioned above are not black listed, implement DKIM. The Symantec's ipremoval tool reports:

• "The IP address you submitted, 93.158.211.31, does not have a negative reputation and therefore cannot be submitted for investigation."
• "The IP address you submitted, 93.158.211.34, does not have a negative reputation and therefore cannot be submitted for investigation."

Could you please check why these addresses are getting blocked?

Thanks,