Skip to main content

Appendix C. Source Routes

Note: Source routing is deprecated and should not be used in modern SMTP implementations.

Historical Context​

Source routing allowed senders to specify the explicit path through intermediate hosts:

Deprecated Syntax:

MAIL FROM:`<@host1.example,@host2.example:user@host3.example>`

This meant: "Route through host1.example, then host2.example, finally to user@host3.example"

Why Source Routing Was Deprecated​

  1. Security Risks:

    • Bypassed security controls
    • Enabled spam and abuse
    • Obscured message origin

  2. Complexity:

    • Difficult to implement correctly
    • Caused routing loops
    • Hard to debug

  3. Obsolete:

    • DNS MX records provide better routing
    • Not needed with modern infrastructure
    • Rarely used since 1990s

Current Recommendation​

Do NOT implement source routing. Modern SMTP should:

  • Use DNS MX records for routing
  • Reject source-routed addresses
  • Return error 550 or 555 if source routes attempted

Example of Rejection​

C: MAIL FROM:`<@relay.example:user@dest.example>`
S: 550 5.5.0 Source routing not supported

Migration Path​

Old source-routed addresses should be converted to standard format:

❌ Old: @relay1.example,@relay2.example:user@dest.example
✅ New: user@dest.example
Last updated on