This letter apologizes for disappointing someone or not living up to expectations.
1234, Main Street
Boston, MA 02123
07 April, 2005
<Recipient Address Goes Here>
I want you to know how awful I feel about letting you down. I know you were really counting on me to <what to do> and I blew it! You must be frustrated and angry with me and I don't blame you. Nothing is more aggravating than having to deal with a flake who can't be depended on. Ordinarily, I am very responsible and reliable. I wish I had a good excuse for why I failed you, but I'm afraid there just isn't one. I hope you'll forgive me for this lapse on my part. Again, I am terribly sorry.
encl: <List of enclosed items goes here>