Catalysis of the transfer of maltose from one side of the membrane to the other. Maltose is the disaccharide 4-O-alpha-D-glucopyranosyl-D-glucopyranose, an intermediate in the enzymatic breakdown of glycogen and starch. This transporter is a porin so enables the energy independent passage of substances, sized less than 1000 Da, across a membrane. The transmembrane portions of porins consist exclusively of beta-strands which form a beta-barrel. They are found in the outer membranes of Gram-negative bacteria, mitochondria, plastids and possibly acid-fast Gram-positive bacteria.