A useful tip is to send yourself the e-mail before sending it to your prospective receiver. This way you can see what your e-mail will look like, especially the layout. This is generally handy if you include an image, or have formatted your message in a particular way. Bullets, numbered lists or outlines tend to get rearranged or unorganized. Also, be careful of copying and pasting content, which can also become shuffled when converted into e-mail form.
Of course you do not need send yourself a copy of every e-mail you are about to send, but for the important ones, this is a great tip to consider!