In email marketing, above the fold refers to content in the preview pane, or space on the window before a subscriber has to scroll down. This is very important to know because most subscribers will never scroll down to finish reading the message unless it's really compelling or interesting. And even then, if the topic doesn't interest them or if they are pressed for time, they will skip reading and move on to the next message in their inbox.
Limited Space
Text space depends mostly on the size of the screen subscribers have, but on average, you can fit about a paragraph (paragraph and a half if not including images) above the fold. This doesn't give you much to work with, but you can get your message across if you write clearly and with purpose. This is why opening paragraphs (along with subject lines) are so important.
Put your best stuff forward by stating the topic and getting right into it. Use bullets, short sentences, and place all links to websites and other online channels at the top of the email message. This way, if subscribers don't scroll down, they can still click on your links.
Images
If the image is pertinent to the text or if graphs and other images help increase click-throughs, try to position them above the fold. You will have to sacrifice some of the text, but if the image helps convert subscribers into customers, then do so. Add secondary images to the rest of the message (below the fold) as necessary.
Video Content
Avoid using flash animation or embedding video content directly into email messages. Many email service providers block this type of content unless permission is granted by the subscriber. Instead, create a link that takes subscribers to your website or video content sharing sites to view the video. This can be a great opportunity to get people to your website, so it's worth losing valuable text space.
Secondary Content
Include the most important information at the top of the page and secondary information below the fold. This does not mean that content below the fold should contain fluffy nonsense - it shouldn't as many subscribers will scroll down, just make sure all pertinent information is included above the fold so more people read it.