The response to a posting on X, formerly known as Twitter, on Dec. 15 that suggested an Xmail email service to rival Gmail would be cool shouldn’t be too controversial. However, when that response comes from Elon Musk himself, you can bet that it will stir things up more than a little. "Yeah. On the list of things to do,” Musk wrote. Here’s why that poses a security threat to billions of Gmail users.