Catalyzes the transfer of the methyl group from monomethylamine to the corrinoid cofactor of MtmC (MtmC1 or MtmC2). Co(I)-[methylamine-specific corrinoid protein] + H(+) + methylamine = methyl-Co(III)-[methylamine-specific corrinoid protein] + NH4(+) One-carbon metabolism; methanogenesis from methylamine. Can form a complex with MtmC (MtmC1 or MtmC2). Belongs to the monomethylamine methyltransferase family.