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Here is step-by-step how to create anonymous recipient relay connector

1. Create a new Receive Connector with the Custom Usage Group
2. For Remote Network Settings default values are 0.0.0.0 -255.255.255.255, where you can add IP address range (for example if you have clients who are talking with exchange on port 25), or/and specific IP Address of the remote server that requires relaying permissions.
3. Once the new Custom Receive Connector is created, go into the properties of this connector
4. Go to the Permission Groups Tab > check Anonymous Users
   1. To activate Anonymous users to use this connector for relaying, you must issue the following command using PowerShell :

Get- ReceiveConnector "Receive Connector Name "| Add-ADPermission - User "NT AUTHORITY\ANONYMOUS LOGON "- ExtendedRights "Ms- Exch-SMTP-Accept-Any- Recipient "

This command is actually adding permission into Active Directory for the Anonymous Logon group, and assign the Ms- Exch -SMTP -Accept -Any- Recipient permission for that group on that connector.