In the Cite Them Right Harvard reference guide, guidelines on referencing email correspondence can be found under the section "Communications", subsection "Public notices and personal communications" and heading "Personal communications".
The guide also states that "You may need to seek permission from other parties in the correspondence before quoting them in your work. You might also include a copy of written communications in your appendices or note where the communication/correspondence can be located (for example, ‘library’). ", so it might worth consulting your tutor/lecturer about it.
