Bitmessage is a P2P communication protocol used to send encrypted messages to another person or to many subscribers. It is decentralized and trustless, meaning that you need-not inherently trust any entities like root certificate authorities. It uses strong authentication, which means that the sender of a message cannot be spoofed. BM aims to hide metadata from passive eavesdroppers like those ongoing warrantless wiretapping programs. Hence the sender and receiver of Bitmessages stay anonymous.