Without knowing the details of how the email is supposed to look, how it is rendering incorrectly, and how you've coded it, it's hard to say what the issue is.
However, I can say that I've personally had a ton of struggles getting Marketo emails to render the way I want them to in Outlook. Email clients all render html emails differently, and unfortunately Outlook is notorious for rendering emails quite poorly. The other frustrating thing is that when you preview the email in Marketo, it renders it differently from how Outlook does, so it doesn't help very much to look at how the email looks in Marketo.
If you give more details about the html in the email, etc, people might be able to be of more help, but for what it's worth, you're not alone in your frustration with this.