Dear …
It is with mixed emotions that I announce to you today that I will soon be leaving my position __(on your team/in your department)__. I am transferring to another __(department/branch/project/etc.)__ within the company. __(Yesterday/Today/Recently/etc.)__, I accepted the __(type)__ position __(at our _____ branch/in our ______ department/etc)__ and will be moving into that new position effective __(date)__.
I feel very fortunate to have been given this great opportunity to advance my career. It will offer me many new challenges and I am looking forward to that with great anticipation. On another note, it will be sad for me to leave all the people I have worked with over the past __(length of time)__. I have really enjoyed working __(on your team/in your department/etc.)__, __(name)__, and I value the relationships I’ve built with everyone there.
I appreciate all the support I received from you and the others and I extend my best wishes to you and the entire__(team/department/etc.)__.
Sincerely,