Dear Felix
It has been my pleasure to serve you for the past [DURATION, ex. two years]. Indeed, I hope that you have been pleased with my service as well.
I am writing to inform you that I will be leaving [FIRM NAME, ex. ABC Co.] to [STATE PLANS, ex. pursue a new career in real estate]. In my absence, I wish to introduce [NAME, ex. Mr. Joseph Johnson], who will serve you in the same capacity as I have.
Allow me again to thank you for the opportunity to work together. If, for any reason, you need to contact me, please do not hesitate to call me at my [new number / home number / etc.], XXX-XX-XX.
Regards
Lydia