So little can you do to offset starting a message with a negative tone. That's a sure way of having it disregarded by most of your readers most of the time, even when ending on a positive note. In fact, they'll likely not get to the positive part. Kicking off a message with "You wrote three glaring errors on your report" or "You made two blunders during the staff meeting" usually sets up a combative situation between writer and reader.
Writing with a positive opening and a negative ending also has its troubles. Beginning with "I hope you're having a nice day" only to conclude with "I expect you to correct these problems immediately" will surely be seen as insincere or thoughtless at best and sarcastic or belligerent at worst.
The best bet is to start and end on positive notes when you want the message to be read in a positive way. So the aim would be to open with "This is the way we would like you to do it" rather than "Don't do it that way," or to close with "I know you'll get this done on time and with quality" rather than "Don't be late and don't make mistakes." Tone matters as much as purpose.
