Yes, MAMMEES is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, M 3, M 3, E 1, E 1, S 1.

Is it a valid word?

MAMMEES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

tropical American tree having wood like mahogany and sweet edible egg-shaped fruit; in some classifications placed in the genus Calocarpum
tropical American tree having edible fruit with a leathery rind
globular or ovoid tropical fruit with thick russet leathery rind and juicy yellow or reddish flesh
brown oval fruit flesh makes excellent sherbet

Also: marmalade tree, pouteria zapota, calocarpum zapota, mammee apple, mammee tree, mammea americana, marmalade plum.