Mariah Carey Burns Dress for Wedding with James Packer

Mariah Carey's sending a harsh -- and expensive -- message to ex James Packer by scorching the actual wedding dress she was going to wear when they walked down the aisle.

Mariah chucks the gown in the fire in her new "I Don't" music vid ... which features YG. Sources close to her say it's no prop either ... it's THE custom $250,000 Valentino she got last year before she called off the Bora Bora wedding to Packer.

Obviously, the wedding never happened ... but luckily the dress was still laying around for the epic scene in the video. Makes sense -- the song's a breakup anthem.

One more shot at Packer -- we're told the video was shot at the former couple's mansion in Calabasas. The burn is real.