Dear …
I am sending this letter today to announce to you that I will be leaving my position of __(name of position)__ with __(name of department/division/company/etc.)__ effective __(date)__. It is with mixed feelings that I make this announcement to you for, though I am __(delighted/happy/honored/glad/etc.)__ to have been promoted to the position of __(name of position)__ within the __ __(name of organization)__, it is with great sadness that I must to say goodbye to a group of people that I have really enjoyed working with.
I will miss the many relationships and friendships I have built over the last __ (#) __ years. It has been a pleasure to be part of such a dynamic group of people who all work together so well in trying to achieve the common goal. Please accept my sincerest thanks to each one of you for your dedication and support. I consider myself very fortunate to have worked with such fine individuals .
As I say farewell, I want to extend my very best wishes to each and every one of you. May you experience continued success both professionally and personally.
Sincerely,