James Packer "is not in his right mind" ... so claim people who Mariah Carey has downloaded with info about her bitter breakup.
Our Mariah sources tell us, "she had to leave him" and he hasn't been "present for her" in a couple of months. They call James "mentally unstable."
Even more serious ... the sources say the last straw for the relationship was that he got "violent" a month ago on a yacht in Greece.
TMZ broke the story ... Mariah claimed Packer was really terrible to her assistant on the boat, and that sent her packing.
Packer's camp tells TMZ ... Mariah's allegations are "simply untrue."