The most common cause of newsletter content expanding past the borders of the newsletter template is adding links and including the full (long) url in the text content block.

Long urls, which contain no spaces and hence do not know how or where to break to a new line, will push out the sides of your newsletter draft.

It is important to NOT add the full URL to outside domains in your text. 

See this FAQ:
How to Add a Hyperlink to Newsletter ~ Edit Link Color

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