Schedule Report not sending to external email address added under "Email addresses". Refer to the below screen shot:
Below Errors in the Email Logs:
If the email mentioned in the Report Email Address section present in the sys_user table, check that the user is "Active" or not. If not "Active" we need to make it active to send email
If the same user is Active but the "Notification" setting is disabled, enable the Notification to that user.
Also refer to the documentation for more details: Schedule a report