I love this extension so far! But i noticed when DKIM is missing rather than invalid, so either no Authentication-Results header or it says dkim=none and either local DKIM check disabled or there's no DKIM signature header, there doesn't seem to be an obvious marker which maybe should be there.
It would help if 1. missing was also always shown (at least as an option), and 2. clear icons would be placed next to the DKIM state, e.g. like this:


(I stole the icons from firefox for the purpose of a quick mockup)