I have a client that just received this message back after sending to one of their clients. This is the first time I have ever seen this. I followed the link that is provided below and I don't think that my setup is wrong, but I could be wrong. I have never had another client report this to me, and I haven't ever seen a message like this. I googled and found a few things about being black-listed, but I checked my server IP and it wasn't on any list.
This is from the returned mail.
host mx01.domain.ca[18.104.22.168] said: 550-Your message was rejected by this user and was not delivered. 550-Reason: Your IP address appears not to be an email server. 550-Protection provided by: MagicMail version 1.2.2 550-For more information, please visit the URL: 550-http://www.linuxmagic.com/best_practices/check_dynamic_reverse_dns.html
All help greatly appreciated.