A receiving network detects fraudulent messages received via a grey route. At a start of a session, a message delivery request is sent to a Short Message Service (SMS) node in the receiving network. This sends a request to a Home Location Register (HLR) of a supposedly originating network for routing information concerning an originator. Upon receiving a response from the HLR, it requests validation of retrieved information, to validate both the originator and the originating network. The SMS node also makes an HLR look-up for a recipient, before performing message delivery.