The primary means of asynchronous long-form text communication (think email) and real-time voice and video communication (think phone calls) both categorize messages into solicited vs unsolicited.
In order for a message to be considered solicited, both the sender and receiver must be registered in the other party's contact list. Registering mutual contact info involves a handshake protocol to ensure that the parties themselves are exchanging this info directly and not, say, a data broker receiving contact info from a 3rd party. Solicited messages are treated with higher priority, have a separate inbox, may trigger audible alerts when received, etc. Besides the obvious case of individual people sharing contact info, one may exchange info when registering with a doctor's office, school, or workplace so that important info can be disseminated. Government entities may offer similar services, and some governments require all accounts set up by their citizens to recognize government messages (disaster alerts and so on) as automatically solicited.
All other messages are considered unsolicited. Either party can at a later time categorize messages from a particular contact as unsolicited, and a sender may flag a message as unsolicited even if they have an established relationship with the receiver.