Unfortunately, when it comes to EDMs (Electronic Direct Mail), Outlook is your worst enemy. Some versions don’t respect padding when a cell’s content dictates the cell dimensions.
The approach that’ll give you the most consistent result across mail clients is to use empty table cells as padding (I know, the horror), but remember to fill those tables with a blank image of the desired dimensions because, you guessed it, some versions of Outlook don’t respect height/width declarations of empty cells.
Aren’t EDMs fun? (No. They are not.)