You to husband: "Let's make an agreement that we only open mail addressed personally to us. It may not be a big deal to you if I open your mail, but I really like to do this for myself."

Start there. You frame it this way it is not so much about him doing something wrong as it is about your own preferences. That may be all it takes.

I remember a similar conversation about laundry many, many years ago. I never put away my husband's laundry. I'd fold it, hang it, etc. but not place it in drawers and closet because I figured how he organized his things was his business. (He thought I should put it away). He, on the other hand, always put mine away (and inevitably in the wrong place). I didn't want him to put my laundry away. We were both used to our own personal habits and actually both not realizing we were getting on each other's nerves. All it took was a quick discussion and a plan on laundry, and the problem was solved.

Sometimes the reasons people do things are just out of habit more than some deep reason. Communication about these little things keeps them from getting bigger.