On my AD/Exchange server, i started getting tons of errors in my Event Viewer, even though i wasn't really noticing any problems with the system.
Event ID 5892 in the Application Log was from the CrmExchangeQueueService and said "The message <message subject>
Event ID 5897 in the Application Log was also from CrmExchangeQueueService and said "An HTTP status of '401 - Access Denied' occurred while attempting to deliver a message. Please check the URL in the registry. Message subject: <message subject>
For me, it turns out it was an easy fix. The password had expired on the account that was running the "Microsoft CRM Exchange Queue Service" on my Active Directory / Exchange server. Once i put in the new password here and restarted the service, i was good to go.