Cleaves alpha 1,2-, alpha 1,3-, and alpha 1,6-linked mannose residues from glycoproteins. Involved in the degradation of free oligosaccharides in the cytoplasm. Hydrolysis of terminal, non-reducing alpha-D-mannose residues in alpha-D-mannosides. Strongly inhibited by swainsonine. Also inhibited to a lesser extent by deoxymannojirimycin (DMM). Belongs to the glycosyl hydrolase 38 family.