oopsie
How about “Dear Abby” by John Prine? It’s a bunch of letters written to Dear Abby, with Abby’s identical response to all letters. For example:
Dear Abby, Dear Abby,
My feet are too long
My hair’s falling out and my rights are all wrong
My friends they all tell me that I’ve no friends at all
Won’t you write me a letter, Won’t you give me a call
Signed
Bewildered
Bewildered, Bewildered,
You have no complaint
You are what your are and you ain’t what you ain’t
So listen up Buster, and listen up good
Stop wishing for bad luck and knocking on wood
Signed
Dear Abby
Not exactly a nationally known act, but they’re pretty big in the mid-Atlantic:
Kelly Bell Band “Dear Maggie”
No one mentioned the intro to You Made Me Love You written especially for Judy Garland to sing?
Uncle Bonsai’s The Letter seems more of an email, but that should count
The Sick Note.
Dear sir, I write this letter to tell you of my plight
And at the time of writing I am not a pretty sight:
My body, it’s all black and blue, my face a ghastly grey.
I write this note to tell you why [Paddy’s] not at work today…
Followed by the long improbable story of an Irish workman trying to haul bricks up to the 14th floor in a barrel. I understand Mythbusters demolished the factual (?) underpinnings, but it’s still a great comic song.