The domestic postal system in France refers to the set of legal, operational, and rate regulations applicable to mail items whose origin and destination are located within French national territory. La Poste’s domestic postal system covers a specific geographic area (I am providing here the data valid as of the early 1970s, including the terminology used at that time; today, the terms DROM (Overseas Departments and Regions) and COM (Overseas Collectivities) are used.):

• Mainland France (including Corsica);
• The overseas departments (DOM): Guadeloupe, French Guiana, Martinique, and Réunion;
• The overseas territories (TOM): Saint Barthélemy, Saint Martin, Saint Pierre and Miquelon, the French Territory of the Afars and Issas, the Wallis and Futuna Islands, New Caledonia, French Polynesia, Clipperton, the French Southern and Antarctic Lands, and Comoros (until 1975, with only Mayotte remaining French);
• Monaco and Andorra: due to bilateral agreements and historical postal enclaves.

Note: Mail sent from mainland France to the overseas territories (or between overseas territories) technically falls under the domestic postal system, but may be subject to additional charges (such as airline surcharges for baggage exceeding a certain weight, for example).