We would love for all of your emails to look equally great everywhere - but the world of email really is something else. Unlike web design, many things we now take for granted are not supported by the most popular email clients or devices - such as certain fonts or inline (CSS in short).
Outlook (windows) is one of the clients not supporting CSS and border-radius and therefore, all round buttons turn up as squares.
If you are using images with round borders, they will also be square in Outlook.
More about this topic:
Why does my email look different in Outlook/Gmail/iPhone?