You are seeing the below error in your Application event log for the Outbound Mail Service:
"An exception occurred during a WebClient request"
This is due to the email that the Outbound Mail Service is trying to send containing an image with is referencing an external path.
If you look in your Incidents, do you have an image in the description field? If this Incident was logged via email for example, and the incoming email contained a signature or a logo it's very possible that this is referencing an external path and therefore will cause this error message when Mail Manager is trying to send any email containing the description of the Incident.
This has been logged as problem 5490. If you are experiencing this issue, please log this with your support provider referencing problem number: 5490 along with giving your impact that this has on your system.