[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].  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-XXXX. 

Sincerely,

[YOUR NAME, ex. Jill Jones]