Dear Felix
It has been my pleasure to serve you for the past [DURATION, ex. two years]. I am confident that you have been pleased with my service as well.
I am writing to inform you that I will be leaving [OLD FIRM NAME, ex. ABC Co.] to join [NEW FIRM NAME, ex. All Co Limited]. I have made this decision [STATE REASON AS IT RELATES TO THE CUSTOMER, ex. due to my displeasure with the support staff at ABC, which I feel prevented me from providing you with the quality service of which I am capable].
I invite you, then, to allow me the pleasure of serving you again at [NEW FIRM NAME, ex. All Co Limited]. I can promise you that you will be doubly pleased with my efforts here. You can reach me at my new number, XXX-XX-XX.
Allow me again to thank you for the opportunity to work together and the for prospect of doing business with you in the future.
Regards
Lydia