A business contact of mine sometimes gets a bounced mail notification. It goes like:
Your IP (185.30.xxx.xxx) is on the BitNinja server security greylist. This means that we experienced malicious attacks coming from your IP and placed it on our greylist due to security reasons. If you have taken the necessary steps to eliminate its source, you can delist this IP on ... ...
He is my only contact who encounters this, likewise I am his only addressee that he get this response from. Is it legit? A google search says yes it is. How would his IP get on their greylist? Will following their link to delist be safe, and effective? My mail is hosted by Crazy Domains, if that is significant.