John, I have accepted an offer with another firm and have decided to tender my resignation as of today. This decision has had nothing to do with the exceptional opportunity you have provided for me here. You and the company have been more than fair with me, and I genuinely appreciate all your support. I wish ABC Company continued success, and I want to thank you for allowing me to be a part of your team. Please feel free to contact me at any time if I can be of further assistance in helping with a smooth transition.
Letters are what get filed and passed around when it comes to explaining what happened.
They are also a means to curtail any ambivalence regarding your decision that might be otherwise perceived by your behavior during this delicate time. The counteroffer Surveys show that eight out of ten employees who accept counteroffers don't complete the following year with their employer. Because the factors that caused you to consider an outside move generally remain in force. Besides, your current employer may lose trust in your loyalty. Never underestimate the value of your perceived integrity in this situation.