A heat exchanger may include a heat exchanger block including a plurality of flat tubes held at a longitudinal end side in a respective rim hole of an associated tube plate. The respective rim hole may have at least two mutually opposite and substantially straight long sides and at least two mutually opposite narrow sides. A brazed connection may be disposed between at least one of the at least two long sides of at least one of the respective rim holes and a corresponding one of the flat tubes arranged therein, and the brazed connection may have an undulating brazing boundary. At least one of the plurality of flat tubes may include at least one partition. The undulating brazing boundary may have a high point that defines an increased thickness in relation to a plane running through two opposite groove bases disposed in the tube plate.