1. A method in a computing system having a processor, comprising: receiving a first message sent from a message-sending server intended for a message-receiving server, wherein the first message identifies a sender address of a sender and a recipient address of a recipient; determining whether the sender address is authorized to send a message to the recipient; and in response to determining that the sender address is not authorized, causing to be sent a second message indicating rejection of the first message; and preventing the message-sending server from sending the first message to the message-receiving server.