That is an easy one - VERY IMPORTANT - poor grammar and improper spelling or usage detracts from the message and if bad enough, becomes the message. I have seen communications stop in a meeting when someone mispronounced a word and again when someone used words that were not in common usage and outside the recognition of the majority of those listening. If this can happen face to face, think what happens when someone is trying to decipher a written message without the author to unscramble it. So in any communication, use the appropriate level of language, use it correctly, and get your message across as concisely as possible.