The answer is, because Outlook is rendering your emails using Word as a browser.
Only a monopoly like Microsoft could get away with that.
Only a monopoly like Microsoft could get away with that.
It's good to remember that Outlook isn't actually an email client. It was bought by Microsoft back in the days before TCP/IP became common. Outlook (still) calls messages "memos". The _first_ thing Outlook does with an RFC-compliant email is convert it into the Outlook "memo" format. There is no option to view the original email message at all. Microsoft simply bolted on a filter to deal with emails. I had hoped that the "Mail" client in Windows 10 would be a real mail user agent, but it's _worse_ than Outlook, without the excuse of predating email and being another program MS bought and rebranded.
What's the problem there? Unless Microsoft forces you to buy a separate Word license to use their email product, who cares if they reuse some of their own code?
If real competition existed senders would send modern HTML mail using CSS and Outlook would not be able to render half of it. But instead developers have to write email to the antique standards used by Word, with layout using tables and without css.
And Microsoft doesn’t care, they get away because Outlook is the monopoly.