5.7.520: automatic external forwarding blocked

What it means

When an inbox rule or mailbox forwarding sends messages on to an outside address, Exchange Online checks the outbound spam filter policy. If the policy blocks automatic external forwarding, the forwarded message is stopped and its sender gets this report, which ends with AS(7555). Since 2021 the default setting, Automatic - System-controlled, blocks external forwarding in new organizations, though some older ones still allow it. Microsoft notes that forwarding can disclose information to attackers, so many admins keep it off and allow it only for users who need it.

Common causes

• A user set up an inbox rule, or an admin set up mailbox forwarding, to an outside address.
• The organization's outbound spam policy blocks automatic forwarding to external recipients.

How to fix it

1. 01Users: ask your admin whether forwarding to that address is allowed; if not, remove the rule.
2. 02Admins: confirm the forwarding is legitimate and was not set up by an attacker.
3. 03To allow it, apply a custom outbound spam policy with automatic forwarding turned on to the users who need it.
4. 04Remote domain settings and mail flow rules can also block forwarding, so check them if messages are still stopped.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• Control automatic external email forwarding from cloud mailboxeslearn.microsoft.com
• Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com