When you enable Symantec Mail Security for Microsoft Exchange 5.0 and later running on Exchange 2003, email stops flowing or becomes very slow. You use a Realtime Blackhole List to screen email senders.
Symantec Mail Security for Microsoft Exchange supports the use of Realtime Blackhole Lists (RBL). An RBL is a list of mail servers that are known to pass spam. Symantec does not provide any RBL service. To use an RBL, you must subscribe to a third-party RBL provider.
Common problems with RBLs
If you suspect a problem with an RBL, contact the RBL provider for support. Symantec Technical Support suggests that you temporarily remove the suspect RBL from the RBL list to verify the problem.