A method of sending protected data from a sender unit to a receiver unit via an intermediate unit. The intermediate unit stores information associated with a certificate belonging to the receiver unit, and information associated with a certificate belonging to the intermediate unit, which has previously been signed by the receiver unit. The intermediate unit receives a request from the sender unit to send protected data to the receiver unit, and so it sends a response to the sender unit. The response includes the information associated with the certificate belonging to the receiver unit, which allows the sender unit to verify that the intermediate unit is authorized to receive data on behalf of the receiver unit. The intermediate unit then receives data from the sender unit that is protected using the information associated with the certificate belonging to the receiver unit for subsequent forwarding to the receiver unit. Having the receiver unit sign the intermediate unit's certificate allows the exchange of credentials to allow a sender unit to send protected data to a receiver unit via an intermediate unit.