A server transmits a message from a sender to a recipient. The server receives from the recipient an attachment relating to the message route between the server and the recipient. The server transmits to the sender the message and the attachment and their encrypted digital fingerprints and expunges the transmitted information. To subsequently authenticate the message and the attachment, the sender transmits to the server what the server has previously transmitted to the sender. The server then prepares a digital fingerprint of the message and decrypts the encrypted digital fingerprint of the message and compares these digital fingerprints to authenticate the message. The server performs the same routine with the attachment and the encrypted digital fingerprint of the attachment to authenticate the attachment the recipient replies to the sender's message through the server. The server records proof of the delivery and content of the reply to the sender and the recipient.