What is the relationship between postbus and mail in the context of the input sentence. Choose an answer from: product/material produced; manufacturer; distributed by; industry; position held; original broadcaster; owned by; founded by; distribution format; headquarters location; stock exchange; currency; parent organization; chief executive officer; director/manager; owner of; operator; member of; employer; chairperson; platform; subsidiary; legal form; publisher; developer; brand; business division; location of formation; creator.

The new plan harks back to Germany's former Kraftpost operation, a more traditional form of postbus that accepted paying passengers on mail services and ran from 1905 on routes that were once the preserve of stagecoaches.

Answer: operator