Legal Literacy has 12 rules for avoiding smoking guns.  The rules are probably good advice for any reason.
In another post, Legal Literacy gives us a thoughtful quote on thoughtless e-mail:

While on the one hand e-mail encourages people to write, on the other hand it discourages people to write thoughtfully.

Mary Mitchell, author of The Complete Idiot’s Guide to Etiquette as quoted in “‘Yours Truly,’ the E-Variations,” New York Times, Nov. 26, 2006.