It sound like they’re hosting at the same place (gandi) where they registered the domain. This is absolutely never recommended, for security purposes. If someone gains access to your hosting account you’ll likely lose all your domain names. Transfer the domains over to a better more secure registrar. Then set up a server or vps and set up as many mailboxes or emails that you need.
Infrastructure best practices have gone out the window, haven't you heard? Most people who use AWS/$cloud_service use it for everything, best practices be damned. Many new projects start their working thinking about how to scale, before making it simple and before having paying users.