A method of tracking mail in a postal sorting system comprising first and second networks of sorting machines with first and second unique identification protocols for uniquely identifying the mailpieces, the first and second networks producing first and second dated tracking data as a mailpiece is passing through the sorting machines, this first and second dated data being associated with first and second unique mailpiece identifiers generated using the first and second protocols, said dated data associated with said first and second unique mailpiece identifiers is retrieved in a data-processing unit for the purpose of measuring a time characteristic of the journey of the mailpiece through the first and second networks, and a match is established between the unique identifiers via digital images of the mailpiece.