Debeaumarché was in charge of the transportation of mail, a position he used to smuggle mail for the resistance. From the summer of 1943 on, he was on the staff of the Action-PTT, a clandestine organisation within the French postal service, and worked with Ernest Pruvost, its national leader, and directly under Simone Michel-Lévy, establishing postal communications with London. When Michel-Levy was arrested in November 1943, Debeaumarché took over as head of the Action-PTT.