As we know the Outlook service doesn't support media queries and has issues with displaying buttons, background images and other elements of an email.
For more details about all the Outlook "pitfalls", click here.
For this specific service, we recommend connecting this control for Outlook buttons in the Appearance/Buttons menu.
This option ensures the most accurate display of your buttons in MS Outlook email clients by inserting a special VML-code element.
Please note, when this option is activated, the email size can be increased up to kilobytes for each added button.
Let's create an email with rounding buttons and see how it looks with the connected Outlook button's control and without it.
Here is our email in the editor, I set 20px for border-radius
Then I exported the email to Outlook and as you can see we have an issue with buttons displaying, they are small and square
Now, let's connect the Outlook button's control and export the email again.
As you can see the buttons displaying properly.
So when you are going to export your emails to Outlook don't forget to connect this control for Outlook buttons.