The website post.lu has a terrible list of postboxes in Luxembourg City. This code puts the addresses into a useful map instead: https://drive.google.com/open?id=1jVL1J6kxpNzRxrro8DM2HlKp2ZM0SyAI&usp=sharing
Addresses taken from https://www.post.lu/en/independants-et-pme/solutions-postales/trouver-une-boite-aux-lettres