Cheers Bob I was thinking the same thing so I looked in the Comodo Logs and there were a lot of Avastsvc (etc) related intrusion events. I went through and changed all the Avast related settings to 'allowed application'. Turned off Comodo's Viruscope and cleared all the application rules. This in itself didn't clear the Mail shield problem.
But I went into Avast and did 'Reset defaults', then did a restart and now its working again.
Don't know why this should happen Avast is in Comodo's 'Trusted Vendors' List.
I will highlight this on the Comodo Forum.