Marketo emails look different on Windows and Mac

Hey everybody,

I have a problem concerning the look of my emails, especially the look of a call to action button.

I created a round CTA button that is aligned center:

That's how it looks in Marketo and also in an HTML editor.

When I send the email to my Outlook account, this is also the look I see on a Mac book.

But when I open the email in my Outlook account in Windows, that's how it looks like:

It's not round anymore and aligned left.

Do you have an idea why this is happening and what I could do to change it?

Kind regards

Christina