Official comment
When using ONTRAmail you can go to Messages >> New Message and select the email you want to copy from the "Copy Existing" drop down:
For the HTML email editor, you can click the Source button in the top left of the editing buttons, copy the email's source code, and then paste it into a new message. That does copy over the email body effectively but you will still need to configure the header information (From, Subject, etc.)
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