Yes, GREENMAIL is a Scrabble word!

Letter values in Scrabble US: G 2, R 1, E 1, E 1, N 1, M 3, A 1, I 1, L 1.

Is it a valid word?

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

(corporation) the practice of purchasing enough shares in a firm to threaten a takeover and thereby forcing the owners to buy those shares back at a premium in order to stay in business