Schema.org currently has EmailMessage for describing a message, but there is no way to model other types of messages or link them together as a conversation.

I propose adding:

• Thing > CreativeWork > Conversation: One or more messages between organizations or people on a particular topic.
• Thing > CreativeWork > Message: A single message from a sender to one or more organizations or people. Properties include:
  • sender: Already exists on ReceiveAction.
  • recipient: Already exists on SendAction.
  • dateRead: The date/time at which the message has been read by the recipient if a single recipient exists.
  • messageAttachment: A CreativeWork attached to the message.

We could then move EmailMessage to be a subtype of Message.