550 5.7.520 Access denied, Your organization does not allow external forwarding.
https://security.microsoft.com/antispam
New > Outbound > +Name > +User > On – Forwarding is enabled
Or edit Default policy using PowerShell:
Enable-OrganizationCustomization Set-HostedOutboundSpamFilterPolicy -Identity Default -AutoForwardingMode On Get-HostedOutboundSpamFilterPolicy -Identity Default