You're probably not going to like the answer: Welcome to HTML-based email, and web (browser- or mail-based) rendering incompatibilities and quirks.
Newer Microsoft Internet Explorer is (much) better than older releases, so see if a newer Windows client or a newer version of Outlook does better.
If you really want to see what's going on here, view the raw HTML that's getting generated, and see what Microsoft wants or uses or expects within Outlook, and send that.
While it is very difficult to construct something like that in Mail, there may be a way you can work around it.
Try this. Create a web page on your Mac that has the appearance at you want. Then, use Safari to "share" that page via e-mail and use the option that includes the entire page. I have no idea if you will get better output, but it is worth a shot.