Build a Website

Hi all, I'm a web designer. I build simple Wordpress websites for small to medium businesses. I have a client who referred me to a someone who needs a free email service built (similar to gmail/yahoomail) for 100000+ clients (as a value added service) that his business has across several African countries. As a web designer, such a project is a bit out of my area of expertise. I'm looking for someone with experience setting up an email service provider website and app for mobile devices. Please, no chancers or anyone trying to outsource it to someone else. I need to be able to see work you have done previously in this area and I need to be able to confirm references for your work. Once vetted, I need you to be able to quickly setup a prototype/demo website I can demonstrate to my client that will allow me do show him that we can create mass email accounts based on data he has in his database. I would be keen on a revenue sharing partnership (not sure on that would work with Freelancer so I haven't set a price for this project yet). Besides my revenue sharing approach, I'm not really sure on how to price such a project so you can ignore the pricing I've set for the project, which Freelancer set as a default.