I've been wondering this for a while - is there a viable business model for email clients? Charging a small one-off fee isn't sustainable really. At the same

I've been wondering this for a while - is there a viable business model for email clients? Charging a small one-off fee isn't sustainable really. At the same time charging a subscription (like SaaS) is unheard of for email clients. Email services on the other hand - a different matter. But even with Google Apps - $50 a year for an email service is really cheap, compared to any other SaaS out there. Wonder why that is, when email is so crucial to every company?