Allowing relay in Exchange 2010 requires the use the command shell. Here are the steps:
1. Open Exchange Management console and go to Server configuration>hub transport. On the right hand side click create a new receive connector. Use custom and name it whatever you like. On the remote network settings step you are going to want to assign a small block or only 1 ip address. Allowing anyone on the internet to relay off of your server will get you blacklisted.
2. Once your connector is created open up the properties of it. Click on the authentication tab and remove all checkboxes. Then click on the Permissions group tab and select only anonymous users.
3. You must now open the Exchange command shell and run this command:
Get-ReceiveConnector “Receive Connector Name” | Add-ADPermission -User “NT AUTHORITYANONYMOUS LOGON” -ExtendedRights “Ms-Exch-SMTP-Accept-Any-Recipient”
You will want to substitue “receive connector name” with teh one that you created.