Added emails not working

Added emails not working

Our main login email is working fine both in and out but our added emails under the same domain name are not. We are getting an error when sending to the email address of

Reporting-MTA: dns;COL004-OMC2S6.hotmail.com
Received-From-MTA: dns;NAM02-CY1-obe.outbound.protection.outlook.com
Arrival-Date: Wed, 14 Dec 2016 14:14:32 -0800

Final-Recipient: rfc822; alex@goodextrading.co.nz
Action: failed
Status: 5.1.1
Diagnostic-Code: smtp;550 5.1.1 < alex@goodextrading.co.nz> User unknown

We are getting this for all email addresses added