You can refer this to debug your application running on cloud
Take a look at the log files of your application: https://docs.mendix.com/developerportal/operate/logs#1-introduction
These should give you a clue on why the Email action does not work.
In most cases there might be something wrong with the settings of your email server.
Take note that you cant send email through localhost in the mendix cloud: https://docs.mendix.com/developerportal/deploy/sending-email#1-introduction