Email Sent is Different than preview

When sending our creating our monthly newsletter, I always use the preview tab and also send out test emails to our staff that both appear fine. However, when I send the real newsletter out to our contacts (me being one of them), the spacing is all off in the desktop view. Extra white space appears where it shouldn't, sizes of images are distorted, and banners don't extend to the end of the page. How is it that the preview and test emails are fine but the final copy isn't? How do I fix this?