This module hasn't been updated for Mendix 5 yet. We don't expose the class com.sun.net.ssl.internal.ssl.Provider from our system bundle because it's not available when not running on a standard Oracle JVM. This app store module should include this itself. You can fix this for now by downloading the relevant jar from http://www.java2s.com/Code/Jar/c/Downloadcomsunnetssljar.htm and putting it in your userlibs folder.
The email module was recently updated to support different jvm's, if you upgrade to that one first, but it is not available yet for Mendix 5. After you migrate to the new module while on Mendix 4, you should be able to migrate the entire project to mendix 5.