It is not unusual to be told by your rental company that you have to pay a traffic fine. Tickets are usually charged to car owners (meaning the rental company), and the rental company is allowed to pass the charge on to you. They may have paid the fine and require you to pay them back.
If the email is really from a reputable rental company they are unlikely to be scamming you (other than perhaps adding a processing charge which you probably agreed to in the fine print). That said it is wise to be cautious. Contact the rental company at an address/phone number that you know to be correct. Check the email was from them. (Though a scammer who knows you rented a car in France through that company already knows a scary amount about you) Ask for details of the offence, and to see a copy of the ticket. You should be able to verify that it is at least somewhere where you were. You may be able to verify that the ticket is real on the appropriate municipality web site.
If you couldn't have been at the place the offence was committed then just decline be to pay. But other than that it's probably not worth fighting.