Not download binaries of the internet and execute them. The mail program the user used was probably written by Apple, one of the biggest companies in the world

Not download binaries of the internet and execute them. The mail program the user used was probably written by Apple, one of the biggest companies in the world. And Mail is probably one of the most hardened and scrutinized against attacks applications in the world. If the user is afraid that Apple mail is faulty or malicious, then installing software from tiny companies is suicide out of fear of death.