Workflows cannot send emails, stuck in PostProcessing state

EmailNotificationDataExchange caught an exception while trying to send an email.　 The email was not sent.　 See the trace immediately following for exception contents.

Second:

System.Web.Services: System.Net.WebException: The request failed with HTTP status 403: Forbidden.

　　 at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)

　　 at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)

　　 at Microsoft.ResourceManagement.WebServices.Mail.Exchange.ExchangeServiceBinding.CreateItem(CreateItemType CreateItem1)

　　 at Microsoft.ResourceManagement.Mail.ExchangeProxy.ExecuteCreateItem(CreateItemType request)

　　 at Microsoft.ResourceManagement.Mail.ExchangeServer.SendNotification(NotificationMessage message)

　　 at Microsoft.ResourceManagement.Mail.NotificationMessage.Send(Int32 timeoutInMilliseconds)

　　 at Microsoft.ResourceManagement.Workflow.Hosting.SendMailWorkItemProcessor.SendMailMessage(MessageContent messageContent, Int32 timeoutInMilliseconds)

　　 at Microsoft.ResourceManagement.Workflow.Hosting.SendMailWorkItemProcessor.ProcessWorkItem(WorkItem workItem)