Why is there a strange space or gap appearing in Outlook?

If the web version of the campaign appears correctly, the spacing issue occurring in Outlook is most likely due to how Outlook renders the specific HTML content. 

Outlook uses Microsoft Word as its rendering engine, which can cause some unintended consequences. It may mean that page breaks are inserted/forced into long tables or before images, or images may be compressed to fit within the height of a typical page. Unfortunately this is a bug with how Outlook renders certain content and sometimes cannot be completely eliminated.

Sometimes the effect can be lessened by editing the content. In general, some recommendations to possibly resolve this issue include:
  • Avoid nested tables where possible
  • Break tables into two or more stacked tables, so that there are no single tables taller than 1700 pixels
  • Break long images into multiple smaller images, so that there is no single image taller than 1700 pixels
  • Add additional horizontal spacing
  • Increase the height of the table rows that precede the breaks