456, X Street, Y Avenue,
[Subject: Normally bold, summarizes the intention of the letter] -Optional-
Dear Andrew Hall,
I am not sure whether you know or not but I have been accepted for a new position at Collins Corporation. I know that I have achieved this position in a large extent to the reference that you provided. I appreciate the fact that you made time to write it on my behalf.
I know that I am not in a position at the moment to return the favor but please ask if there is anything I can do in return. Many thanks once again for the assistance you have given me.