If letter is returned unclaimed - that is the postman has left a remark 'unclaimed' on the envelope containing your letter - then it is a good service and even if the letter is not physically received by the addressee, by law the addressee is deemed to have knowledge about your letter
so no need to do anything further
however if you wish, you can scan the office copy of your letter and the envelope with the unclaimed remark and email those scans to the addressee, if you have their email address