The purpose of sending a service apology letter is so as to express regret over the inability of a service provider to maintain provision as per the Service Level agreement, and also undo any impression that may serve to portray that it was trying to abdicate on its responsibility.
The best letter is one that comes before the actual failure or error is set to begin and seeks to instill confidence and imply accountability on clients. The letter should steer clear from technical or business jargon and be straightforward in its content and also maintain a feeling of regret over the hassle caused as a result. The words should serve to reassure the clients that such occurrences are not common and that they will not recur. Any letter that is sent after the error should be clear and simple in explaining what happened in the briefest of words and at its end attempt at reining in the pardon of the client in an attempt at self redemption.