My Customer wants me to show the Outlook Emails(or Email subject only) from his Office365 OWA to his SharePoint Online (intranet Portal).
I tried to call the OWA in I-frame but it showing error, means not allowed.
First I'll suggest you to read this good article on how to setup OWA in I-frame. IF it doesn't resolve your issue, you can also retrieve the Email subjects with the help of REST call and them format it in HTML to display it on your Page using CEWP. But for this to take place outlook and SharePoint should be on the same domain. Kindly read this article