In order to accomplish what you want, you will need to keep relaying enabled but use the Routing Restrictions option in the Routing tab of the IMS. Once there, you will want to configure a range of address in the Hosts and Clients with these IP addresses option.
It is a bit tricky to configure a range, as it does not let you specify a starting address and ending address. Instead, you must enter and IP address with its subnet mask, which essentially gives relay rights to the entire subnet. If you want to specify an individual IP address, then you must use the syntax: 188.8.131.52 255.255.255.255. In addition to this, you do not want to include IP addresses for any of your Exchange servers.
For more information on configuring and testing controlled relays see Knowledge Base article 836500, "Relaying and unsolicited commercial e-mail in Exchange Server 5.5".
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