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.

yep, that's my question. can I just create any .html file with html/css/pictures/links, and insert the code into an email using an email client (in my case, Apple's Mail.app on MacOS X Tiger 10.4 ...