In your 2013 Exchange Management Shell run the following 3 commands replacing "MessageStatsServiceAccount" with your messagestats account.
1. New-ThrottlingPolicy MessageStatsPolicy
2. Set-ThrottlingPolicy MessageStatsPolicy -RCAMaxConcurrency Unlimited -RcaMaxBurst Unlimited -RcaRechargeRate Unlimited -RcaCutoffBalance Unlimited -EWSMaxConcurrency Unlimited -EWSMaxSubscriptions Unlimited -CPAMaxConcurrency Unlimited -EwsCutoffBalance Unlimited -EwsMaxBurst Unlimited -EwsRechargeRate Unlimited -MessageRateLimit Unlimited -RecipientRateLimit Unlimited
3. Set-Mailbox "MessageStatsServiceAccount" -ThrottlingPolicy MessageStatsPolicy
