If you have created project in higher version , you cannot open that in lower version. So create a new project in lower version and try to implement your requirement. Thanks
What you need to do is create a new project in a Mendix 8 version. Then import the email module and then export that module again. You now have a mpk file. Then import that file in your Mx 9 project. Due to being a beta version you can not directly import this module yet.
You can not open your project in a lower Mendix version.