Extra space added to HTML e-mail when sent from Outlook to Gmail

There is no fix for this, but there are ways to limit and hide the size of the separation. I'll refer to it as separation, but it is really the top cell expanding due to Outlook applying the mso-normal p tag. Here is a related article.

Between tables creates a bigger gap (about 15px) than between table rows (2px). It is more complex, but try and put everything that you can't afford separating into one big table. Colspans and rowspans can be complex, but they work if set up correctly.

To hide the separation, wrap all the tables in a master-content table with the bgcolor set. For example, make one big content pane set to white. When sending from Outlook, all your child tables will then move, but the gaps will remain white, stopping unwanted lines appearing in your email body.

A trick for footers - As the last child table of your main white panel, when this separates, it will create a white line underneath it. Either remove the footer table from the main panel or set the cell bgcolor to the same as your email background to hide this separation.