Oh, thanks for detailed computer details. Your inbox is stored in one DBX archive and avast doesnt know how to remove one file from this archive. I want to say, one your email contains beagle virus in inbox, it will not active itself, if you dont open it directly :). Maybe you could seek the problematic email in your inbox and remove (use “message with attachments” filter). Maybe in future, avast will handle dbx/mbx/… files.