A heat exchanger has a header plate with a plurality of openings and a plurality of tubes each having an end portion inserted into one of the plurality of openings in the header plate. Each of the plurality of tubes has a generally rectangular cross-section and a longitudinal axis. An end portion of at least some of the plurality of tubes is stiffened by a respective stiffening part. Each stiffening part includes two reinforcing elements positioned opposite each other abutting an inner surface of the tubes. Each reinforcing element also includes a back portion and two side portions extending from the back portion and a connecting member connecting the side portions of the reinforcing elements.