A system for immunizing a computer network against adverse effects caused by the receipt of a corrupting message. Each message transfers into a protocol-based controlled environment for a specific recipient where message criteria determine whether the incoming message is deemed to be a valid or suspicious message. Transmission criteria determine the final message disposition. If the message is valid, it is delivered to a recipient computer system in the network. If the incoming message is suspicious, the message is isolated in the controlled environment where the transmission criteria may provide remote access to the recipient.