I'm not sure what is classed as a 'signature' from the point of view of this module. I'm assuming, based on my testing, that it expects at least a single dash or underscore underneath the email content? My company emails do not include this, but have my name, job role and then some 'small print' about the confidentiality.
Basically, I'm assuming that if there's no visible separator between email content and signature, the whole this is classed as 'visible content' ?