The original server setting was x.com and now I have changed over to y.org. I have update the server setting under account to point to y.org, however every now and again the server changes back to x.com. I suspect this typically happens when I shut down and restart Outlook. Additionally I get a untrusted certificate request for x.com popping up.

Additionally I have cleared all references to x.com from Utilities>Key chain access. For good measure I have also cleared my dns cache and my browser cache.