You can “click in the Subject and change that” and also “Add a Note” to the message, but to edit the received message in the body, you have to Reply or Forward the message which then creates a draft as per @Gary post below in the following thread.
You can “click in the Subject and change that” and also “Add a Note” to the message, but to edit the received message in the body, you have to Reply or Forward the message which then creates a draft as per @Gary post below in the following thread.