To get more information on Client Throttling policies, you can refer the following articles:

General information about client throttling policies
More Throttling changes for Exchange Online
Exchange Online EWSMaxSubscriptions throttling budget calculation 
EWS Throttling in Exchange Online
Effects of Throttling on your Deployment in Exchange 2010 SP1
EWS best practices: Understand Throttling policies
Exchange Server 2010 – Client Throttling Policies