Hydrolyzes 1,4-beta linked polysaccharide backbones of mannans, one of the major hemicellulose components in hardwoods and softwoods. Shows very high activity against mannohexaose but not against mannopentaose and smaller mannooligosaccharides. The major products released from mannooligosaccharide hydrolysis are mannose and mannobiose. The reiterated 40 AA domain is involved in binding the cellulase-hemicellulase complex. Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. Consists of a catalytic N-terminal domain linked to a reiterated non-catalytic C-terminal domain. Belongs to the glycosyl hydrolase 26 family.