If you are receiving the following error:

Delivery has failed to these recipients or groups:

Your message wasn’t delivered because the recipient’s email provider rejected it.

And then when you go into the Message Trace in Office 365 you see this error:

Reason: [{LED=550 5.7.520 Access denied, Your organization does not allow external forwarding. Please contact your administrator for further assistance. AS‎(7555)‎};{MSG=};{FQDN=};{IP=};{LRT=}]

Then you might be experiencing the new Security Change Microsoft Implemented on October 5, 2020, which by default blocks forwarding to external recipients on a Mailbox.

1. To resolve the issue, go to https://protection.office.com and go under Threat Management > Policy > Anti-Spam Policy > Outbound spam filter policy. Then click EDIT POLICY.

2. Then under the Outbound spam filter policy Edit screen, click “On- Forwarding is enabled” under the Automatic Forwarding section. Then click SAVE.

Your messages to any Office 365 mailbox now that has an external recipient in the forward will now process successfully.