Override for default apps?

Is Evolution and Thunderbird the only supported email clients for default apps? I've recently installed Newton and tried making this my default for email but it never shows up as an option. Settings within the app don't appear to help either.

It depends on how the developers classified the app. What i mean is, like evolution, you may have to choose Newton in calendar instead of email to set it as default. It may not be listed in all possible places.