  Double perovskites Sr$_2$FeMO$_6$ (M=Mo and Re) exhibit significant colossal
magnetoresistance even at room temperature due to the high Curie Temperature
(419K and 401K). However, such a high Curie Temperature is puzzling, given the
large separation between magnetic elements (Fe). Moreover, with M=W, the
electronic and magnetic properties suddenly change to insulating and
antiferromagnetic with the N{\'e}el temperature of only 16$\sim$37 K. Based on
detailed electronic structure calculations, a new mechanism is proposed which
stabilizes the strong ferromagnetic state for M=Mo and Re and is passivated for
M=W.
