The Avast mail shield acts like a man in the middle in order to scan the secure connection between you an the mail server. This alert is showing that.
You can confirm this by disabling the mail shield and trying the connection again, the message should not pop up.
Totally unhelpful reply. I too need to get at the Avast CA. I too could disable Avast to prove that's the problem... but what is needed is a solution!
