Hi Krwelliver,
Currently, we received several posts about the issue “Error 18500” or “HTTP error. The Server cannot fulfill the request” in Outlook for Mac 2011.
Our product team is aware of this issue and working hard to fix it. When there is any update about this issue, we will post the information here.
We are sorry for any inconvenience this case has caused. Your patience and understanding will be highly appreciated.
Best Regards,
Bourne Zhang