Error:
Your message wasn't delivered because the recipient's email provider rejected it.
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)
Apparently it is some new policy Microsoft implemented but back in before October?
Some say to turn on Forwarding in the EAC for each mailboxes, that's too tedious.
The solution appears to be what someone suggested from the first link here:
By DennisDepoorter-9036
As an admin go to: https://protection.office.com/antispam
Open Outbound Spam filter policy and click Edit policy
Set "Automatic forwarding" "ON"