I have started just using M. Like, "Dear M. Lastname". I saw it being done in French and just adopted it for English too. No one's complained yet. (Have also seen Mx. but figure that could be confusing.)

Yeah, I know! That's why I was surprised when someone addressed me as "M." (while I don't present as such, traditionally) in French. So, I thought "that's neat!" and picked it up.