Apple Mail (on both macOS and iOS) is designed to follow a principle called "send what you compose." The email client determines the format of an outgoing message based on the content it detects in the compose window, not on a blanket setting.
Coding for email is a fine art as there are so many variables that one needs to take into consideration, none the least of which are the different mail clients and what they do or do not support. Each is unique. As the third most used mail client in the world, Outlook has idiosyncracies that are of course different to those of Gmail and the 2016 release has its own set of requirements. Here are…