User:David MacQuigg/Sandbox/Email agents

Actors and Roles

Actors include Users and Agents
Agents may play more than one role
Typical roles include Transmitting, Receiving, Forwarding, and Delivery.
A Border shows where there is no prior relationship between Agents.
--> Direction of mail flow (no statement as to relationship)
==> Direct relationship between Actors (e.g. a contract)
~~> Indirect relationship (e.g. both directly related to Recipient)
A/B Roles A and B both played by the same Actor

Typical Systems

Basic setup with four actors

|--- Sender's Network ---|           |-- Recipient's Network -|
                                /
Author ==> MSA/Transmitter --> / --> Receiver/MDA ==> Recipient
                              /
                           Border
          

Simple forwarding is quite common

          |-------- Recipient's Network ---------|
     /
--> / --> Receiver/Forwarder ~~> MDA ==> Recipient
   /
 Border

Chain forwarding should be discouraged

          |------------ Recipient's Network ------------|
     /
--> / --> Receiver ~~> Forwarder(s) ~~> MDA ==> Recipient
   /
 Border

Open forwarding must be banned

     /                   /    |-- Recipient's Network -|
--> / --> Forwarder --> / --> Receiver/MDA ==> Recipient
   /                   /
 Border              Border

Roles and Responsibilities

Author
- Originate messages
- Provide a password or other means of authentication

MSA - Mail Submission Agent
- Authenticate the Author
- Manage Author accounts

Transmitter
- Spam Prevention
  - rate limits, content analysis, alerts
  - respond to spam reports
  - maintain reputation
- Authentication
  - RFC compliance
  - IP authorization (SPF, SID, CSV, ...)
  - signatures & key management (DKIM ...)
  - Return Address validation code
- Process SMTP Rejects

Receiver
- Block DoS
- Authenticate Sender
  - HELO, Return Address, Headers, Signature
  - reject forgeries
- Assess reputation
  - whitelists
- Filter spam
- Add authentication headers
- Manage Recipient accounts/options
  - whitelisting, blacklisting, filtering, blocking, forwarding
- Process spam reports, DSNs

Forwarder
- Authenticate upstream Agent
- Set up forwarding to downstream Agent
  - check RFC compliance
  - set up authentication records
  - submit forwarding request, wait for approval
- Manage Recipient accounts
  - maintain database of forwarding addresses
  - suspend account when a message is rejected
  - communicate w Recipient re  "      "
- Maintain reputation as a trusted Forwarder
  - certifications
- Process SMTP Rejects

MDA - Mail Delivery Agent
- Authenticate upstream Agent
- Sort and store messages
- Provide access for Recipients
  - POP3, IMAP, Webmail
- Manage Recipient accounts/options
- Relay spam reports to Receiver (or don't accept them)

Recipient
- Set up accounts with each Agent
- Select options in each account
- Report spam to Receiver

Mediator
- Receive - Process - Resend automatically
- Acts as an Agent, but
- Classified as a User for simplicity