The contract between the courier and the sender is a postal service contract. According to Article 124 of the Contract Law, for contracts that are not expressly provided for in the specific provisions or other laws, the provisions of the general provisions of this Law shall apply, and the specific provisions of this Law or the most similar provisions of other laws may be referred to. The contract in this case falls under the above provisions, and there are no provisions in the specific provisions, so it is an unnamed contract. The type of unnamed contract is mainly determined based on the content, performance method and subject matter of the contract, because the courier and the sender use the service item of sending specific mail items as the subject matter of the contract, and its content and performance method are that one party pays a certain fee and the other party provides certain labor services. Therefore, the nature of this unnamed contract should be characterized as a service contract and classified as a postal service contract. According to the determination of the civil cause of action of the Supreme People's Court, the dispute between the two parties is a postal service contract dispute.