This is a configuration issue of outlook.
The clients probally have plain text mode enabled:
Tools...
Options...
Mail Format...
Compose in this message format = Plain Text or Rich Text
If this setting is set to HTML, then the page will look as you want it to.
WARNING: Even in HTML mode, certain HTML characteristics are not enabled unless in "internet mode" so you cannot have javascript or load external cascading style sheets for example.
Internet mode is only enabled when view a message by clicking:
View...
View in Internet Zone
Because you cannot change the client's outlook options, you will probally need to provide both a plaintext version of your e-mail, as well as a HTML version.