Never assume anything.
Don't make promises you are not sure you can keep.
If in doubt, leave it out. Go with your gut feeling.
If there is time, have someone read your email before you send it. A second pair of eyes can spot errors that you are unable to see because you have been staring at the text so long. This is common practice among journalists when writing stories.
If you suspect there may be legal problems with an email, there probably are – run it past your manager or the legal department.
When writing a name or a number put (X) after it and only remove it after you have double-checked it.
Check all URLs by calling them up. A hyperlink that is wrong or does not work is bound to annoy the reader.
Don't believe everything you read on the Internet. Many websites show little commitment to accuracy.