You are looking for more information about preventing your system from relaying spam outbound. We notify you if we observe that you have sent out spam emails and the Support team may have already stopped relaying outbound emails from your compromised server. We recommend that you check the following points to help prevent outbound spam relay about outbound email relay and possible outbound mail flow interruption.
Ensure that inbound connections on port 25 are only accepted from our IPs. The full list list of ranges can be found here: http://images.messagelabs.com/EmailResources/ImplementationGuides/Subnet_IP.pdf
Ensure that you do not have any test accounts or demonstration accounts with generic or easily guessed passwords.
Disable guest accounts. (Guest accounts usually have the user name "Guest" and a default password. If you disable the guest accounts, you reduce the risk of an attacker breaking into the system.)
Open relaying in Microsoft Exchange
Open the Microsoft Exchange Manager.
Navigate to the Virtual Server section, Administrative Groups, Servers, (your mail server ), Protocols, SMTP sub-tree.
Click Properties on the Action menu.
Click the Relay tab.
Ensure that the checkbox that is labeled Allow all computers which successfully authenticate to relay, regardless of the list above is NOT checked.