Is it o.k. to end letters etc with the words ‘Best Regards’ or ‘Highest Regards’ when addressing Non-Muslims?
It is allowed for a non-Muslim to write these words. For example, if someone has good intentions in a sense that he wants the other person to be blessed with the true religion, or he wants him to be saved from the hellfire then in this case to use some good words in the meaning of guidance would be allowed.
Original Source Link (from Way Back Machine archive)