If you are working or doing some migration work for a 40 years old company, chances are you bump into one or few legacy applications that don’t support SMTP authentication but it is still being used and will not be retired any soon. These application sends out email for notifcation.
To resolves this, we need to allow Exchange Receive Connector to allow relay without any form of authentication. Here’s how to do it form Exchange Management Shell(EMS).
Get-ReceiveConnector -Identity “Anonymous Relay Connector” | Add-ADPermission -User “NT AUTHORITY\ANONYMOUS LOGON” -ExtendedRights “Ms-Exch-SMTP-Accept-Any-Recipient”
As a best practice, make sure that you only allow connection from specific IP address. This can be done from Exchange Management Console (EMC) > Microsoft Exchange On-premises (your server name) > Server Configuration > Hub Transport > Receive Connectors