ErrorTooManyRequests

Description of the error code "ErrorTooManyRequests"

Domain: Exchange 

Code: ErrorTooManyRequests

Type: Temporary error (Please try again later. If multiple temporary errors occur, contact support.)

Description:  Exchange received too many requests. 

Possible solution: The EWSMaxConcurrency parameter might be set too low.
This issue might be solved by assigning ApplicationImpersonation to the connecting account, which will allow it to operate with a copy of the resource budget of the resource it is accessing.