One of the most important provisions in the Geneva Convention are the rules concerning the right of prisoners to maintain correspondence with their relatives … The detaining State may censor the mail of detainees, so long as censorship is not used as a pretext for withholding mail from prisoners. 

Prisoners-of-war must be allowed to exchange letters with their relatives and receive parcels of food, clothing, medicine, religious artefacts, literature and study requisites. The imprisoning country is permitted to censor the prisoners’ post as long as this is not used as an excuse for withholding their post from them.