Remote Server returned ‘550 5.7.520 Access denied, Your organization does not allow external forwarding. Please contact your administrator for further assistance. AS(7555)
With an email box set to auto-forward to an external email. But you received an error message like the above.
Here is the solution:
If the admin wants external email forwarding, the admin must change the default Outbound spam filter policy under Anti-spam settings.
Access to Security & Compliance admin center > Threat management > Policy > Anti-spam > Outbound spam filter policy > Turn on Automatic forwarding.