@alnxln . The organisation you are sending to has detected that the ip address which you currently have was used in the past by someone else for malicious attacks. They "greylisted" it with Bitninja; presumably not as bad as blacklisting.
The link takes you to Site Protection by Bitninja website where you can clear the greylist on the ip address. Then try to resend your message.
Or, just reboot your router which will get a new public ip address and resend message.