I have multiple users that have forwarding set up either on their Outlook application or on the Exchange 365 server. When messages are auto forwarded they end up with an NDR similar to this:
Delivery has failed to these recipients or groups:
email@domain.com
Your message wasn't delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept e-mail from certain senders, or another restriction may be preventing delivery.
The following organization rejected your message: DM2PR08MB334.namprd08.prod.outlook.com.
Generating server: Domain.com
email@domain.com
DM2PR08MB334.namprd08.prod.outlook.com #<DM2PR08MB334.namprd08.prod.outlook.com #5.7.1 smtp;550 5.7.1 Unable to Relay. ATTR11> #SMTP#
Problem has been happening since June.