456, X Street, Y Avenue
[Subject: Normally bold, summarizes the intention of the letter] -Optional-
Dear ANDREW HALL,
You can probably imagine how worried I was when I returned to work after four weeks away. I was fearful that there would be a huge pile of work in my in-tray, work that would take me weeks to clear. The fact that you stepped up and did my work in addition to your own is a great relief to me.
I now see you as a role model of a perfect colleague and will seek to do likewise when any colleague is absent from our team. Thank you once again.