Assume Nothing

Never assume that the recipient is familiar with the thinking behind the E-Mail at hand. If not, make sure that you let the person know the background and the issues. When following up, don't assume everyone remembers everything you've said earlier. If you've got any worries that an acronym, term, or reference is going to elicit a confused moment, it is better to explain it rather than having the recipient confused. Ask yourself if you are hiding anything from the reader unintentionally.