A server transmits a message from a sender to a destination address. During transmission, the server and the destination address have a dialog constituting an attachment, via a particular one of SMTP and ESMTP protocols, concerning the message, the server and the destination address. The message passes through servers between the server and the destination address. This passage is included in the attachment. Verifiers are provided for the message and for the attachments. The verifiers may constitute encrypted hashes of the message and of the attachment. The sender receives the message, the attachments and the verifications from the server before authentication and transmits the message, the attachments and the verifiers to the server to obtain authentication by the server. The server operates on the message and the message verifier to authenticate the message and operates on the attachments and the attachments' verifier to verify the attachments.