Disabling automatic forwarding to external addresses in Office 365 is being rolled out in phases with details communicated via Office 365 Message Center posts. To help administrators prepare for these changes have them modify policies ahead of time to ensure there is no disruption to their users.

When a message is detected as automatically forwarded and the organizational policy blocks that activity a Non-delivery report (NDR) will be generated back to the end user. The report will indicate the message was not delivered. The NDR will have the following format:

5.7.520 Access Denied – Your administrator has disabled external forwarding – AS(XXXX)

Enable Automatic External Forwarding for All Mailboxes

Log in to your Office 365 Security & Compliance portal for Microsoft 365and choose Threat management > Anti-spam or head directly to the Anti-spam settings page here: https://protection.office.com/antispam

Click the down arrow next to Outbound spam filter policy ‎(always ON) and click the Edit policy button.

Click the down arrow next to Automatic forwarding, then choose On - Forwarding is enabled from the dropdown. Click Save.