In Mac Mail, have you ever experienced that some (not all) of your e-mail messages do not wrap? Yet the same message in the iPhone and iPad wrap just fine. This is a problem with Mac Mail, and it has been around for a long time. You can search the Internet for many stories of users who are experiencing this problem. But there seems to be only one solution.
You can actually read the message normally in Mac Mail if you just hit the reply button, whether you intend to reply or not.