• Use the persons name flatly. Don't start out with "Dear". Don't say Mr. or Mrs. If you want to come off as extra-snobby, you can also use "Ma'am" or "Sir". It should look like this: "David,". Short and curt.
• Hit the news head on and right out of the gate. Don't apologize, don't explain, don't try and ease into it. Be blunt, be straightforward and boldly announce what it is you're trying to say. It should look like this: "I will no longer be working for your department." Don't ask for permission or confirmation. Just say it.
• Make a passive aggressive statement. There is a fine line between passive aggressive and plain old aggressive. If you want to be passive aggressive, you'll have to be sneaky about it so that if someone else were reading the email they'd have no idea it was meant to be mean. It's better to focus on I statements and speak about yourself as much as possible. Here's an example: "I have decided to work for another department where I feel I will learn more related to my interests and at a pace better suited to me." There, you've just insulted David's department, teaching abilities, and also commented that he's too slow for you without even mentioning him.
• Make another passive aggressive statement. Try complimenting someone else's role in your email (and CC them on it). This will be an extra one-two punch. For example, "I've been in touch with Mike Smith from the other department (CC him). He has been gracious enough to allow for the switch and has been very helpful throughout." You're telling David that you've finally found someone who can help and what a relief it is that it isn't David.
• Push them off. The person you've emailed is likely going to be upset at this point and may be tempted to email you back with an equally passive aggressive email. Close the possibility of this by pushing them off with a closing statement. Drive this point by CC'ing another person. Like this: "If you have any questions or further concerns, please feel free to email Amanda with HR (CC her). She has been key in helping the transition between departments." You're telling David that you aren't open to a response and to please email someone else if he takes issue with it.
• Stamp it out. This is the last chance you have to be passive aggressive so make it count. Don't compliment your person or say thanks for their time, don't do anything except for close what you've just said. Like this: "I will be in Friday to collect my things. I wish you the best of luck in your future endeavors. Please let me know when you are free to sign my last time sheet." This is like a middle finger to David.
• End it (or don't). You can sign the email "Respectfully" or "Sincerely" knowing that it's a lie but if you really want to be a dick, just use your name.
• Other Notes. Keep the email as brief as possible, laying out only the facts. Never falsify information. Keep a copy of the email for future records. CC the appropriate people so that you have an audience (this will further humiliate whoever you are dealing with). But again, make it subtle enough that no one will take notice of the passive aggressiveness except for the person who is receiving the email. All in all the email will read like this:

David,

I will no longer be working for your department. I have decided to work for another department where I feel I will learn more related to my interests and at a pace better suited to me. I've been in touch with Mike Smith from the other department. He has been gracious enough to allow for the switch and has been very helpful throughout. If you have any questions or further concerns, please feel free to email Amanda Hirsch with HR. She has been key in helping the transition between departments. I will be in Friday to collect my things. I wish you the best of luck in your future endeavors. Please let me know when you are free to sign my last time sheet.

Lindsey