Session Date/Time: 23 Jul 2025 07:30

SML Session Meeting Minutes

Summary

The SML (Structured Mail) working group met on Wednesday morning in Madrid to discuss the core draft, trust and security considerations, and implementation progress. Key topics included IANA registrations, MIME structure handling, full representation semantics, and a new experimental fork of Thunderbird Mobile demonstrating SML functionality. The group also reviewed a trust and security draft and conducted an adoption poll.

Key Discussion Points

Implementation Progress

Thunderbird Mobile Fork: Demonstrated working implementation with smart link previews and structured data handling for music sharing, recipe extraction, and authentication codes
Fork available on Google Play Store with APK distribution planned
Examples showed enhanced rendering of Apple Music links, New York Times cooking newsletters, and Apple authentication codes

Core Draft Technical Issues

IANA Registrations and Registries

Two proposed registries:
- Schema lookup registry for email-relevant vocabularies (schema.org, email-specific schemas)
- Documentation registry for email-specific vocabulary concepts
Need for clearer draft language and IANA desk consultation
Registries intended for developer reference, not real-time client downloads

MIME Structure and Body Part Headers

Proposed content-purpose: machine-readable header to prevent structured data from appearing in legacy clients
Discussion of signed/encrypted JSON-LD handling (JOSE-based signatures)
Debate over wrapper media types vs. direct MIME type handling
Concerns raised about signing subparts vs. whole messages

Full Representation Semantics

Clarification needed for distinguishing full message vs. partial representation
Complex MIME structures with multiple multipart/alternative sections create ambiguity
Legal footer handling for business compliance (Germany, Ireland, France requirements)
Proposal to use schema.org publisher field for structured legal information

Email Lifecycle Handling

Reply and forward mechanisms for structured emails
Error handling for structured responses (invalid identifiers, authorization issues, server errors)
Message updates using supersedes header
Consideration of splitting reply functionality into separate draft

Message Flags and Headers

has-structured-data flag similar to existing has-attachment flag
Debate over utility for web clients vs. mobile/resource-constrained clients
Discussion of additional flag for full representation indication
Questions about flag accuracy and advisory nature

Trust and Security Draft

Descriptive approach focusing on running code behavior rather than prescriptive requirements
Security concerns catalog for implementers
Trust models including:
- User contact lists as privileged senders
- Contractual relationships between MUA authors and senders
- Payment processor transaction ID matching for enhanced trust
Concerns raised about payment processor barriers to entry and transaction ID entropy requirements

Decisions and Action Items

Immediate Actions

IANA Registration: Complete formal IANA registry documentation with assistance from IANA desk
Core Draft: Address MIME structure ambiguities and full representation semantics
Trust and Security: Conduct working group adoption call based on poll results (6 in favor, 2 against)

Technical Decisions

Maintain descriptive approach in trust and security draft
Consider splitting reply functionality into separate document
Retain advisory nature of structured data flags
Focus on semantic rather than visual representation for full representation concept

Follow-up Items

Develop real-world examples for payment processor trust model
Clarify error message routing (envelope sender vs. reply-to)
Add sender perspective to message update scenarios
Review interaction with anti-spam mechanisms

Next Steps

Core Draft Finalization: Address remaining MIME structure and representation issues to prepare for working group last call
Trust and Security Adoption: Proceed with adoption call and incorporate feedback
Implementation Feedback: Gather input from Thunderbird Mobile fork users and other implementers
Registry Development: Work with IANA to establish formal registry procedures
Vacation Notices: Review deferred structured vacation notice draft (not discussed due to time constraints)

The meeting concluded with strong implementation progress demonstrated and clear technical issues identified for resolution before advancing the specifications.