Someone said :
Is it illegal to deliberately possess a copy of a sensitive and confidential letter not addressed to that person?
Someone holds a copy of a confidential letter. The letter is supposed to be confidential between two parties. But one party gave the copy to a third party.
Not illegal on the face of it, unless as Kev says, there's a prior agreement but even that is a civil rather than criminal law matter. One of the parties passed on the contents of the letter. You have not committed a crime by receiving it. It's not theft, at worst it's a civil breach of confidentiality.
If you are really concerned, seek legal advice. Place the document in the hands of a legal advisor. Unless the author is Kevin Sowdon, you should be fine.