I use a third-party email system for one reason: The spam protection here is bare bones, little more than a basic SpamAssassin setup with little configuring. I’d suggest you consider the approach taken by the ConfigServer MailScanner Service. It’s actually an add-on to cPanel, but greatly expands the spam protection making it near as good as the dedicated email hosting systems. But maybe you can get some ideas from them, or help them develop a version for your email system.
Sure, maybe you want to push everyone to Gmail, but some of us aren’t enamored of trusting our stuff to Google.