There are many reasons why an email may not be rendering correctly and it is not possible to describe all of them due to there being many different versions of Outlook, mobile clients, web clients, etc.
However, there are 2 features in the DnD that are expected to not work on Microsoft's desktop based Outlook clients.
Rounded Corners - In buttons and any other element that allows for setting rounded corners, this setting will not take effect in Outlook so the corners will be square instead.
Background Images - These will not display on Outlook at all. However, it is important that this be taken into consideration in the design itself. Any text overlaid onto the image should be readable without the background image loading (even for general accessibility reasons). With this in mind, ensure that there is enough contrast between the text colour and the background colour of the element.
If your issue is not described above, please reach out to your account manager at firstname.lastname@example.org.