A real problem could be that the "denial of mileage credit" letter is often a standard form-letter irregardless of one’s particular situation. I received the same letter as the OP saying that I had failed to send in my ticket stubs. I had, and happily kept a copy. When I called MP, a very helpful CS agent pulled the record and discovered the problem with the name (the reservation on a partner airline had my middle initial which is not on my MP account). But the letter denying my missing credit request made no reference to this at all, but said that I had failed to submit the BP stubs. Because of the CS agent, my mileage posted. UA could have been much more helpful (and generated much more good will if the letter denying mileage credit was (a) accurate and (b) didn’t immediately jump to “it must be your fault.”