Dear Mr./Ms. Bossname,
I can’t believe I’m resigning! It’s thanks to you and your mentorship that I’m in a position to move on from XYZ Company. My last day will be May 30.
I am so grateful for your guidance, from training me in Widget Repair to ensuring that I had every opportunity to pursue Advanced Widget Studies thanks to the company’s educational reimbursement program.
Thanks also for your letter of recommendation. I think it played a big part in me getting the job.
You’re the best boss ever! I will miss your leadership and the whole team at XYZ Company.