Dear Boss,
As you may know, I am leaving the company today and would like to convey my sincere thanks to you. It was a real pleasure working with you and the company. I had a wonderful experience in your firm and was able to learn many new things from my seniors and coworkers.
I am really thankful to you as you have helped me and supported me all the way long. In my days in the company, I have learnt many things from you. The way you handled the team and as a result gained more confidence to handle certain situations
As my contact number remains the same, you can feel free to contact me any time.
Regards,
John