Sounds reasonable to me. 
You might want to include a brief definition for mixed content too just in case you reach someone who does not know what it is. Something like: "images, links, scripts, etc. with http:// in their addresses". Without the definition, I've found here that most people will usually ask (even though one click on the checker will provide a complete explanation). Some won't even go that far. Since you're coming to the recipient in this case, I feel that minimizing the effort he/she needs to exert is more likely to lead to an active outcome. Don't hold your breath though. 
I contacted a major (and controversial) news organization (One America News Network) a few weeks ago about exactly this issue who has yet to take any action (or even send a response).