If you are receiving the following error in your email :
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 content you read this error:
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)'
You might be experiencing the new security change Microsoft implemented on late 2020, which by default blocks forwarding to external recipients on a mailbox.
To resolve this issue, go to https://protection.office.com and go under Threat Management > Policy > Anti-Spam Policy > Anti-spam outbound policy > Edit Policy :
The future forward messages to any mailbox will now be process successfully by O365. Cuation : the settings can take several minutes to take effect.