Cargomail

Introduction

Despite the importance of the email infrastructure, the entire ecosystem still relies on a vintage architecture and protocol design that is more than 40 years old. While conceptually sound as a means of communication, the Internet mail system is structurally incomplete and functionally deficient.

Cargomail introduces a revised Internet mail architecture that includes a new Resource Handling Service (RHS) alongside the existing Message Handling Service (MHS). The RHS stores digital resources from one user and delivers them to one or more other users, creating a virtual server-to-server exchange environment. This architecture allows for the exchange of large volumes of documents, images, videos, and audio files of unlimited size. Additionally, it enables advanced anti-spam protection that uses a challenge-response mechanism to ensure that only legitimate messages are delivered to the recipient. The challenge-response mechanism requires a certain, variable amount of computational effort on the sender's part, making bulk spamming costly. It means that anyone can send you an email without restrictions, but those who intend to spread spam in large quantities will find it difficult and time-consuming.

Revised Internet Mail Architecture

This section proposes a revised version of the Internet Mail Architecture, IETF RFC 5598. The revised architecture separates the mailbox from the email address and uses an authorization layer that allows individuals to control access to their digital resources across Administrative Management Domain (ADMD) boundaries through email messages. This extra authorization layer, together with the transport layer, comprises servers, agents, and protocols working collectively to create the RHS—a complementary service to the MHS. The RHS extends the capabilities of the current Internet mail system and has the potential to address spam and attachment issues more effectively than the MHS-only email system.

Fig. 1. The revised Internet mail architecture

Key Points

Each email object consists of a placeholder message and associated external resources (message bodies) stored on the Resource Server (RS) of the respective mailbox service. The placeholder message also functions as an access control list for its external body resources. The information flow illustrated in Figure 1 includes the following key points:

• The body resources owned by the author stored on the RS of the origin mailbox service are temporarily shared with recipients by creating a placeholder message which also acts as an access control list. Following a successful sharing process, the placeholder message is sent to each recipient through the MHS. This placeholder message stores, in its headers, the URL of the origin mailbox service and the cryptographic hash values of the referenced body resources (see Appendix A for a placeholder message example).

• After receiving the placeholder message, the recipient's Message Transfer Agent (MTA) stores it in the mail queue. From there, the placeholder message is delivered to a content filter using an SMTP connection. The content filter adds a header containing the destination mailbox service URL to the placeholder message and filters out the list of referenced body resources based on predefined rules. The resulting list is then stored in the mailbox service queue. The placeholder message is sent back to the mail service, where it is further processed in the standard way, see Postfix After-Queue Content Filter.

• The Resource Fetch Agent (RFA) running on the destination mailbox service retrieves the URL of the origin mailbox service and the cryptographic hash values of the referenced body resources from the filtered list stored in the destination mailbox service queue. Using the GRIP authentication mechanism, the agent attempts to fetch the external body resources from the RS of the origin mailbox service. After successful authentication, the data is fetched and stored on the RS of the destination mailbox service. Finally, the user agent downloads the relevant data from the RS of the destination mailbox service and reconstructs the original message according to the placeholder message source.

Anti-Spam Protection

Challenge–response authorization

TBD

Appendix A—Placeholder Message

Here is a placeholder message in JSON format with external bodies accessible via content-addressed URIs.

{
  "headers":
    {
      "X-Origin-Mailbox-Service-URL": "https://foo.com/mbx",
      "X-Destination-Mailbox-Service-URL": "https://bar.com/mbx",
      "From": "Alice Sanders <alice@foo.com>",
      "Subject": "Meeting",
      "To": "Bob Sanders <bob@bar.com>",
      "Cc": "Carol <carol@bar.com>, Daniel <dan@bar.com>",
      "Date": "Tue Sep 19 20:52:05 CEST 2023",
      "Message-ID": "<b07d0cdf-c6f4-4f67-b24c-cc847a4c2df4@foo.com>",
      "X-Thread-ID": "<68fb9177-6853-466a-8f7d-c96fbb885f81@foo.com>",
      "Content-Type": "multipart/mixed"
    },
  "parts":
    [
      {
        "headers": { "Content-Type": "multipart/alternative" },
        "parts":
          [
            {
              "headers":
                {
                  "Content-Disposition": "inline",
                  "Content-ID": "<aSQnmlBT6RndpDnwTSStJUVhlh9XL9_y2QXX42NhKuI>",
                  "Content-Type":
                    [
                      "message/external-body; access-type='x-content-addressed-uri'; hash-algorithm='sha256'; size='42'",
                      "text/plain; charset=UTF-8"
                    ]
                }
            },
            {
              "headers":
                {
                  "Content-Disposition": "inline",
                  "Content-ID": "<Y_ION3g8WQuqGzhsDlVrhAgQ0D7AbXu9T-HSv3w--zY>",
                  "Content-Type":
                    [
                      "message/external-body; access-type='x-content-addressed-uri'; hash-algorithm='sha256'; size='109'",
                      "text/html; charset=UTF-8"
                    ]
                }
            }
          ]
      },
      {
        "headers": { "Content-Type": "multipart/mixed" },
        "parts":
          [
            {
              "headers":
                {
                  "Content-Disposition": "attachment; filename='cargomail_architecture.svg'",
                  "Content-ID": "<1pzyqfFWbfhJ3hrydjL9jO9Qgeg70TgZQ_zpOkt4HOU>",
                  "Content-Type":
                    [
                      "message/external-body; access-type='x-content-addressed-uri'; hash-algorithm='sha256'; size='52247'",
                      "image/svg+xml"
                    ]
                }
            },
            {
              "headers":
                {
                  "Content-Disposition": "attachment; filename='Cargomail.pdf'",
                  "Content-ID": "<6G6Mkapa3-Om7B6BVhPUBEsCLP6t6LAVP4bHxhQF5nc>",
                  "Content-Type":
                    [
                      "message/external-body; access-type='x-content-addressed-uri'; hash-algorithm='sha256'; size='153403'",
                      "application/pdf"
                    ]
                }
            }
          ]
      }
    ]
}

• X-Origin-Mailbox-Service-URL: The origin mailbox service URL.
• X-Destination-Mailbox-Service-URL: The destination mailbox service URL.
• Contentd-ID: Cryptographic hash value of the referenced body resource.