A resignation letter should always be addressed "To whom it may concern", you may just want your letter of resignation to speak to a certain supervisor, but your letter may get "escalated" to higher-ups and you might like to appeal to them also. Using the phrase "It is with a heavy heart that I... " Heavy heart doesn't imply anger or satisfaction and while you might not be real happy about your situation, demonstrating specific emotion could be misunderstood. Keep it fairly short but do try to explain yourself. Do not mention names of others who you feel have hurt your situation. If your employer is interested in further details they will ask and if they are not then that demonstrates their indifference. If you are leaving a job where it was all good and you are moving onward and upward, then stay very positive. Always remember - the people you meet on the way up may still be there when you are on your way down. And Good Luck!