I just looked at your Reverse DNS and everything here looks perfect. When I looked at your monitors the event history shows that they keep going down with these errors
Unable to Connect to SMTP Host
DNS Resolution Failed
It is my guess that your network is going down and so we are unable to reach your DNS servers that you are hosting yourself. Each of our lookups will attempt to resolve your DNS from the root so that we can detect any DNS problems.
You can confirm this by changing your monitors to use the IP's directly instead of the hostname. If I am correct, they will continue to go down on the monitors, but it will only have "Unable to Connect to SMTP Host" and the "DNS Resolution Failed" will go away. If this is the case, I would suggest that you have at LEAST secondary DNS servers that are hosted for you.
You can see this warning if you run a DNS check against your domain here: http://mxtoolbox.com/DNSCheck.aspx