Transfers mannosyl residues to the hydroxyl group of serine or threonine residues. a dolichyl beta-D-mannosyl phosphate + L-seryl-[protein] = 3-O-(alpha-D-mannosyl)-L-seryl-[protein] + a dolichyl phosphate + H(+) a dolichyl beta-D-mannosyl phosphate + L-threonyl-[protein] = 3-O-(alpha-D-mannosyl)-L-threonyl-[protein] + a dolichyl phosphate + H(+) Protein modification; protein glycosylation. Belongs to the TMTC family.