Thank you for reaching out.
'Message' resource supports -
- Adding your own data as custom Internet message headers. Add custom headers only when creating a message, and name them starting with "x-". Once the message has been sent, you cannot modify the headers. To get the headers of a message, apply the $select query parameter in a get message operation.
- Adding your own data as custom properties as extensions.
You can create a draft of the Forward/Reply message. You can then update or send the draft.