In previous versions of Microsoft Outlook (the classic app), you could view the HTML code of an email by opening the email, right-clicking on it, and selecting “View source” from the context menu.

Better safe than sorry. Some file types stripped by my SMTP proxy are useless to the users (com, js, chm, cmd, cpl, bat, and 30-40 others). Useful ones that I strip are EXEs (no choice), eml and html.