One of the most common polite mistakes I see in business writing is this: a message that ends "Thank you again. . . ." without thanking the reader at the beginning.
If you are in the habit of thanking the reader again at the end of your messages, be sure you said thanks somewhere previously. Otherwise, your politeness will sound canned and look silly. That would be a shame when your intention is to connect sincerely with your reader.
Thanks for reading!