A mail pouch that is sealed shut and that is used to carry communications between a legation and its home office WordNet 3.0 & Open English WordNet

A diplomatic bag, also known as a diplomatic pouch, is a container with certain legal protections used for carrying official correspondence or other items between a diplomatic mission and its home government or other diplomatic, consular, or otherwise official entity. Wikipedia

A container with letters etc from representatives of a given country to another country, which has immunity from examination by customs officials. Wiktionary

A diplomatic bag. Wiktionary