a question about articles

[peppy_man]

'I've just read ( a / the ) referral letter from Dr. Jones.'

In this example, which article is correct, 'a' or 'the'?
I guess 'the' is the right answer, but would like to hear native teachers' opinions.

[Anglika]

Rather depends on the situation. Either is correct

"I've just read a referral letter from Dr Jones relating to Mrs Brown's case."

"I've just read the referral letter from Dr Jones about Mrs Brown's mammogram."