[NAME, COMPANY AND ADDRESS, ex.
John Smith
XYZ Inc.
1234 First Street
Suite 567
Anycity, Anystate  85245] 

Dear [NAME, ex. John Smith],

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. AllCo 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. AllCo Limited].  I can promise you that you will be doubly pleased with my efforts here.  You can reach me at my new number, XXX-XXXX. 

Allow me again to thank you for the opportunity to work together and the for prospect of doing business with you in the future.

Sincerely,

[YOUR NAME, ex. Jill Jones]