I have received a similar email after booking the Protea Hotel Franschhoek.
I think it is a legitimate request, but I don't see the need for this as they have my credit card on file on my profile...
I have been in contact with the hotel in the meantime. The mail is genuine indeed, they confirmed they received the money.
It does seem like a bad integration of the Marriott system and the Protea system where deposits are taken from the protea system and aren't managed through marriott.com. (In some cases even for bookings where marriott.com doesn't mention any deposit at all...)