This paper proposes an adaptive and reliable message delivery protocol for mobile objects. The protocol uses a mailbox-based scheme, which associates each mobile object with a mailbox while allowing the decoupling between them. It provides location-independent message passing and overcome message loss caused by mobile object's mobility. It also reduces the reliance on home location sever and relaxes the constraint on mobile object's mobility. The protocol is suitable for different mobility and communication patterns by choosing different mailbox migration frequency properly. Its applications include mobile agent system, mobile Internet and short message service.