Error in server side synchronization - Async service is stopping automatically

Hi Experts,

Async service is getting stopped automatically when server side synchronization email router profile is created.

below is the trace log captured. Kindly advise how to solve this issue.

[2016-06-17 10:04:52.628] Process:CrmAsyncService |Organization:00000000-0000-0000-0000-000000000000 |Thread:    5 |Category: Platform.Async |User: 00000000-0000-0000-0000-000000000000 |Level: Error |ReqId: 00000000-0000-0000-0000-000000000000 | FaultToleranceBehavior.Execute  ilOffset = 0x18
>Exception while executing async service: MSCRMAsyncService - System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.Exchange.WebServices, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.
File name: 'Microsoft.Exchange.WebServices, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'
   at Microsoft.Crm.Asynchronous.EmailConnector.MailboxConfigurationValidator.ValidateServerLocation()
   at Microsoft.Crm.Asynchronous.EmailConnector.MailboxConfigurationValidator.ValidateAssociatedEmailServerProfile()
   at Microsoft.Crm.Asynchronous.EmailConnector.MailboxConfigurationValidator.Validate()
   at Microsoft.Crm.Asynchronous.EmailConnector.MailboxTestAccessOperationBase.Validate()
   at Microsoft.Crm.Asynchronous.EmailConnector.MailboxOperationBase`1.Execute()
   at Microsoft.Crm.Asynchronous.EmailConnector.MailboxOperationCommand.InternalExecute(MailboxAsyncEvent asyncEvent)
   at Microsoft.Crm.Asynchronous.AsyncEventHandlerCommand`1.Execute(IAsyncEvent asyncEvent)
   at Microsoft.Crm.Asynchronous.AsyncEventExecutionManager`2.ExecuteHandler(IAsyncEventHandlerFactory handlerFactory)
   at Microsoft.Crm.Asynchronous.PoolHandler.ProcessAsyncEvent(IAsyncEventExecutionManager asyncEventExecutionManager)
   at Microsoft.Crm.Asynchronous.Operations.AsyncEventOperation.<.ctor>b__2(IServiceOperation operation)
   at Microsoft.Crm.OperationBehaviors.FaultToleranceBehavior.Execute(ServiceOperationAction operation, IServiceOperation operationParameter)
   at Microsoft.Crm.Operations.MonitoredOperation.Execute()
   at Microsoft.Crm.Asynchronous.ThreadPoolQueueExecutionEngine.InvokeNextOperationInThreadPool(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()