Mail.protection.outlook.com #550 5.7.1 Service unavailable; Client host blocked using Spamhaus;

When user in our domain send mails to domains on O365, they get a DSN.

BY2FFO11FD035.mail.protection.outlook.com
rejected your message to the following e-mail addresses:<o:p></o:p>

mail.protection.outlook.com #550 5.7.1 Service unavailable; Client host [xxx.xxx.xxx.xxx] blocked using Spamhaus; To request removal from this list seehttp://www.spamhaus.org/lookup.lasso

We check Spamhaus.org and our IP and domain are clear.

Any ideas?

Thanks in advance

http://nextadmin.blogspot.com