Spam emails that come from unknown senders are not blocked by Greylist.
Possible causes for this filter to malfunction:
To verify the operation of Greylist:
Step 1: Confirm that Greylist is enabled
From the Greylist properties, ensure that Enable Greylist is selected. For more information refer to Greylist.
Step 2: Verify excluded addresses
Step 3: Use esentutl.exe to ensure that the Greylist database is not corrupted.
For more information refer to http://go.gfi.com/?pageid=ME_esentutl.