I am wondering if it is a security issue that our Proofpoint-server doesnt like a ordinary mailserver send a response back if you try to send to an nonexisting user in the domain?
Hi LuminusR,Kindly check if the settings below is enabled?Email Firewall > Recipient Verification > General > Message Does Not Contain A Valid Recipient > Reject the message and return the following Thanks.Best Regards,Jayson