550 5.7.520 Access denied, Your organization does not allow external forwarding. Please contact your administrator for further assistance. AS(7550)
Enable Automatic External Forwarding for All Mailboxes
Log in to Microsoft 365 Defender as a Microsoft 365 administrator and choose Email & collaboration > Policies & rules > Threat policies > Anti-spam policies or head directly to the Anti-spam settings page here: https://security.microsoft.com/antispam
If you do not see those options or no policies display on that page, the Microsoft 365 user you have used to log in does not have sufficient permissions to make these changes. Make sure you are logging in as an administrator for your account. Note that the policies you see in your own admin may differ from those shown here, as these are only the default policies.
Click on Anti-spam outbound policy (Default) and scroll down to click the Edit protection settings link at the bottom of the sidebar.
Find the section called Forwarding Rules, and the dropdown list called Automatic Forwarding Rules. Pull that list down and choose On – Forwarding is enabled. Click Save at the bottom. 
Enable Automatic External Forwarding for Individual Mailboxes
Log in to Microsoft 365 Defender as a Microsoft 365 administrator and choose Email & collaboration > Policies & rules > Threat policies > Anti-spam policies or head directly to the Anti-spam settings page here: https://security.microsoft.com/antispam
If you do not see those options or no policies display on that page, the Microsoft 365 user you have used to log in does not have sufficient permissions to make these changes. Make sure you are logging in as an administrator for your account. Note that the policies you see in your own admin may differ from those shown here, as these are only the default policies.
Click + Create policy and choose Outbound.
Give your new outbound spam filter policy a Name and Description.
Click Next and search to find the user account you want to allow to forward, i.e. the email account that you are forwarding to Help Scout, which will display under the Users field after you select it.
Click Next again, scroll down to the Forwarding rules section, and click the dropdown under Automatic forwarding rules. Choose On – Forwarding is enabled, then click Next.
Review the settings on the last screen and click Create to create your new outbound policy for the specified user(s).
