Exchange Online - Remote server returned 550 5.7.520 Access denied, Your organization does not allow external forwarding

Office 365 - How to fix "Remote server returned 550 5.7.520 Access denied" error by creating a custom Outbound Anti-spam policy to allow external forwarding

Remote server returned 550 5.7.520 Access denied, Your organization does not allow external forwarding

Error when trying to forward emails externally from Office 365

Your email couldn't be forwarded from email@domain.com to another email address.
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

• Forwarding emails externally is blocked by default in the Office 365 outbound spam filter policy
• Blocking external forwarding stops attackers from getting access to emails when a mailbox had been compromised
• If you want to allow external forwarding for a mailbox, you'll need to create a custom Outbound Anti-spam Policy
  

Enable external forwarding in Office 365 using an Outbound Anti-spam Policy

Logon to the Microsoft 365 Defender portal
https://security.microsoft.com

Email & collaboration - Policies & rules

Threat Policies

Anti-spam 

Create policy - Outbound

Name: Allow external forwarding

Include these users - select the mailbox you want to allow external forwarding for

Automatic forwarding rules: On - Forwarding is enabled

Click Create

Click Done

Reference:

https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/outbound-spam-policies-external-email-forwarding

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