Words on a computer screen look different than on paper, and usually people find it harder to read things on a screen than on paper. (I know several people who even print out their email to read it.) The screen's resolution is not as good as paper's, there is sometimes flicker, the font may be smaller, and/or the font may be ugly. Your recipient's email reader may also impose some constraints upon the formatting of the mail, and may not have the same capabilities as your email software. This means that good email page layout is different from good paper document page layout.

Shorter Paragraphs
Frequently email messages will be read in a document window with scrollbars. While scrollbars are nice, it makes it harder to visually track long paragraphs. Consider breaking up your paragraphs to only a few sentences apiece.

Line Length
Some software to read mail does not automatically wrap (adjust what words go on what line). This means that if there is a mismatch between your software's and your correspondent's in how they wrap lines, your correspondent may end up with a message that looks like this: