Related Rule
Belgium
Practice Relating to Rule 125. Correspondence of Persons Deprived of Their Liberty
Belgium’s Law of War Manual (1983) provides: “Not more than one week after his arrival at a camp, even if it is a transit camp, every prisoner of war should be enabled to write, directly to his family and to the Central Prisoners of War Agency, a card of a special model.” It adds that prisoners of war should be allowed to send and receive cards. 
Belgium, Droit Pénal et Disciplinaire Militaire et Droit de la Guerre, Deuxième Partie, Droit de la Guerre, Ecole Royale Militaire, par J. Maes, Chargé de cours, Avocat-général près la Cour Militaire, D/1983/1187/029, 1983, p. 47.
Belgium’s Specific Procedure on the Prisoners of War Information Bureau (2007) states that the tasks of the PWIB (Prisoners of War Information Bureau) include: “collecting and registering the following information concerning each prisoner of war in the power of the Belgian armed forces: … the address to which correspondence for the prisoner may be forwarded”. 
Belgium, Structure et fonctionnement du Bureau de Renseignements sur les prisonniers de guerre, Procédure spécifique, Ministère de la Défense, 2007, pp. 6–7, § 7(a)(1).
In the case of Belgian prisoners of war in the power of the enemy, the Specific Procedure provides:
If the Belgian PW [prisoner of war] has not expressed his refusal to have his family informed of his situation, a letter will be sent to the address of the person recorded as “to be informed.” This letter shall contain the following information, in so far as it is available: … the address to which correspondence for the prisoner may be forwarded. 
Belgium, Structure et fonctionnement du Bureau de Renseignements sur les prisonniers de guerre, Procédure spécifique, Ministère de la Défense, 2007, p. 11, § 12(c)(1).