Outlook
Outlook 2007, or more specifically the Word engine, has a document length limit of 23.7 inches, or around 1800 pixels. When the email hits that length, Outlook inserts a page break to aid with document printing. This can be avoided by using stacked tables that are less than 23.7 inches in height. A new text boundary is created every time you create a new table element so one table might be 10 inches and another might be 15 inches. As long as each individual table does not exceed the height constraint you won’t end up with automatically generated text boundaries.
Gmail
Gmail clips emails that have a message size larger than 102KB, and hides the full content behind a View entire message link. Removing spaces and carriage returns within your code can reduce file size tremendously.