Classic and Lightning report refresh subscriptions work a little bit differently, but neither is capable of sending a message to an arbitrary email address outside Salesforce. You can email users and groups who have Salesforce accounts for delivery to email addresses that are set on their Salesforce profiles.
Per Schedule a Report for Refresh,
You can send reports only to email addresses included on Salesforce user records.
Per Subscribe to Get Refreshed Report Results in Lightning Experience,
When the subscription emails the refreshed report to each recipient, it sends to the email address set in Settings | Email | My Email Settings. If no email is set in My Email Settings, then the refreshed report is sent to the recipient’s email address set on their Salesforce User record.
The easiest solution is likely to be a forwarding rule on your corporate email server. Running the report in Apex and generating and sending emails in code would be substantially more complex.