Reverse DNS is not a valid Hostname in the Linode platform

Reverse DNS is not a valid Hostname: Ok, all my settings are correct but I still can’t seem to track down why MXTools states my rDNS is not valid.

I seems that this issue is restricting email delivery.

My rDNS setting is set to

Please reply like you speaking to a child or a golden retriever. I’m new to this. While I am smart I have little experience in this arena. I thank you in advance for your input and assistance.