Envelope-content splitting

Envelope-content splitting (ECS) is a messaging technology in which the message headers (envelope) are sent through the messaging system independently of the message content. The core of ECS was developed in 1996 and U.S. patent was granted in 1998.
Implementation
An ECS-enabled messaging client consists of two parts: sending and reading. An implementation may also include encryption capabilities, for securing data on the content server.
The message body, along with the sender's credentials, is sent to a content server via a secure channel. Optionally, the message body is encrypted using a symmetric key encryption technology such as AES, before being sent to the content server. The content server returns a reference to the content in its database. Any attachments are sent in turn to the content server, which returns a reference for each. If the content is encrypted, the same key is used for all message parts. The mail client adds ECS-specific fields in the mail header identifying the content server name and port, as well as a field for the (space-delimited) content references and an optional encryption key field. When the mail message is sent, only the message envelope is sent to the recipient, plus an optional default message for the benefit of clients that do not support ECS. The recipient mail client looks for ECS header fields and if it finds them, uses them to make requests to the content server. The message body is retrieved and displayed to the user. Attachments are retrieved on demand.
Comparison with traditional technologies
Since the message content is not sent directly to the messaging recipients, the sender maintains ownership of the content indefinitely. In particular, he or she may change the content at any time.
Mail intermediaries, including SMTP, IMAP and POP servers, as well as mail exchange servers, are unable to access the message content, so privacy of the message in transit is ensured.
Sending only envelopes requires less space in recipients' mailboxes and delivery to large distribution lists is much faster, because content delivery only happens on demand.
Since the actual message content does not pass through any mail server, attachments may be arbitrarily large and are not limited by any preset limit imposed by the SMTP host software.
Encryption schemes are simpler because the encryption key is included in the messaging header, so multiple keys and complex key management are not needed, as in typical public-key cryptography implementations.
Messaging address "spoofing" is made much more difficult, because additional credentials are needed in order to send an ECS message. DMARC provides an alternative technique for preventing spoofing, but DMARC requires a complex implementation activity by mail carriers.
One implementation of ECS-enabled products is provided by Chiaramail Corp.

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