A message is rejected with the following error:
Unknown address error 550-'5.7.1 Recipient rejected (R4)'
- Add the user/aliases affected by this to the User List
- Found under Management tab > User Management
- Change the Unknown Users setting
- Found under Management > Inbound Filtering > Spam Handling Settings > Unknown Users
- Note: BLOCKED is the recommended option
When Unknown Users is set to block and the user is not listed in the Control Panel, then email to that user is rejected.