I have lately been getting activeSync Heartbeat warning from our 2010 Exchange server. I usual task is to try and make our servers run as smooth as possible, ensure all systems are updates, and run everything to as close as possible to best practices.
Now, couple things to note. We run a pretty tight infrastructure, and activeSync isn't the only service being handled by our TMG server. Below is a very basic idea of how activeSync flows.
Palo Alto --> TMG --> Exchange
For anyone wondering about TMG's EOL and possible replacements, they can check out those recommendations here and here.