On Exchange 2013/2016 You may Receive an Error like :
[Eas] The number of outstanding requests for guard TargetBackend(“ServER_Name.fqdn.local”) has exceeded the max limit 150. Current request will be rejected.
The Solution :
On all CAS servers:
In \Exchange Server\V15\FrontEnd\HttpProxy\sync\web.config, add this key in the node.
In \Exchange Server\V15\FrontEnd\HttpProxy\rpc\web.config, add this key in the node.
After this change recycle the MSExchangeSyncAppPool and MSExchangeRPCProxyAppPool in IIS manager (if you have more then one server I recommended to take a delay of 5 min between Servers ).