Show if a mail was transmitted and received with TLS

[tomsommer]

Like so: https://support.google.com/mail/answer/6330403?hl=en

By checking Received: headers

[pimlie]

Thanks for the suggestion. Allthough I like the idea, the main issue I have with this isssue is that its actually a bit out of the scope for this plugin. This plugin is currently only about the Authentication Status, hence its name.
Adding support for other/more validation techniques are maybe better suited in a different plugin, what do you think?