The keys used to secure this link are discarded after use. The message would be further encrypted inside the link between the secured mail hubs by the sending client, using longer-term keys. An attacker cannot eavesdrop on this ciphertext even if he has the private key needed to decrypt it. To compromise the security of the system, the attacker must breach security on one of the mail hubs, avoid detection for the period he is able to obtain ciphertext, and still obtain the recipient's private key. Ideally, secure SMTP would be combined with short-term encryption keys to provide forward secrecy between mail clients and secure mail hubs.
Internal links are generally more secure, however, than Internet connections.
It would also allow users more control over the keys providing forward secrecy, maintaining security even after a succesful attack on a secure mail hub. The three methods outlined all increase the security of encrypted messages, by reducing the information revealed upon the compromise of a private key. Short-term encryption key pairs limit the damage to the lifetime of the compromised key, and can be used with existing software. One-time keys reduce vulnerability to a very small number of messages, but require minor modifications to software. Secure SMTP uses new keys for every session, but relies on the security of the external mail hub.