information:classification, structural:summarization
Physical services

The postal service (snail mail) physically delivers letters using a best-effort delivery approach. The delivery of a certain letter is not scheduled in advance – no resources are preallocated in the post offices. The service will make their "best effort" to try to deliver a message, but the delivery may be delayed if too many letters suddenly arrive at a postal office or triage center. The sender is generally not informed when a letter has been delivered successfully, unless one pays for this premium service. When the addressee is unknown the message might be returned to sender. Or the letter might be definitively lost or destroyed without notification.