A method of processing mailpieces for sorting a plurality of batches of mailpieces into sequence in a postal sorting machine based on delivery points of delivery rounds, in which method the mailpieces are sorted in the postal sorting machine by batch as a function of various sorting plans associated respectively with the various batches of mailpieces, each sorting plan associated with a batch of mailpieces defining an ordered list of delivery points of one or more delivery rounds, said ordered list being saved in the memory of a control unit controlling the sorting machine, in which method batches of mailpieces are assessing in the control unit before the machine sorting to detect those that will under-use the sorting capacities of the postal sorting machine, i.e. those for which the mailpieces will fill the sorting outlets of the postal sorting machine during a sorting pass to an insufficient extent, and in which method if an under-use of the sorting capacities of the postal machine by a batch of mailpieces is detected, an optimized sorting plan optimizing the use of sorting capacities of the postal sorting machine is computed for said batch of mailpieces, which method is characterized by the following steps: computing in the control unit during the assessing of each batch of mailpieces a cumulative value of the number of active delivery points and comparing said cumulative value with a threshold value to detect the batches of mailpieces that will under-use the sorting capacities of the postal sorting machine; • aggregating the batches of mailpieces detected as under-using the machine by physically concatenating two or more batches of mailpieces that will under-use the capacities of the sorting machine to constitute one or more aggregated batches of mailpieces in such a manner that the overall cumulative value of the active delivery points with the aggregated batches of mailpieces is close to but less than the threshold value; • computing in the postal sorting machine, for each aggregated batch of mailpieces, an associated optimized sorting plan resulting from the aggregating; and • feeding each aggregated batch of mailpieces into the postal sorting machine and controlling the postal sorting machine in such manner as to sort the mailpieces as a function of the optimized sorting plan computed for said aggregated batch of mailpieces. computing in the control unit during the assessing of each batch of mailpieces a cumulative value of the number of active delivery points and comparing said cumulative value with a threshold value to detect the batches of mailpieces that will under-use the sorting capacities of the postal sorting machine; • aggregating the batches of mailpieces detected as under-using the machine by physically concatenating two or more batches of mailpieces that will under-use the capacities of the sorting machine to constitute one or more aggregated batches of mailpieces in such a manner that the overall cumulative value of the active delivery points with the aggregated batches of mailpieces is close to but less than the threshold value; • computing in the postal sorting machine, for each aggregated batch of mailpieces, an associated optimized sorting plan resulting from the aggregating; and • feeding each aggregated batch of mailpieces into the postal sorting machine and controlling the postal sorting machine in such manner as to sort the mailpieces as a function of the optimized sorting plan computed for said aggregated batch of mailpieces.