Mailing Template Tables
This comes down to the difference between how email clients and web browsers render HTML.  This article from Litmus gives a good overview.

There are two solutions:


Develop emails with email-centric design.  Since Outlook is the worst offender, design for Outlook.
Install the Mosaico extension.  Its drag-and-drop email template builder takes the guesswork out since the blocks are already Outlook-friendly.