smtp mail server is ok ,when i stop cisco ACS server,it does not send e-mail to my mail-box,why?

Hi, Got a server running ACS v3.3 talking to a SecurID ACE server. When I set a user up to use SecurID, ACS always reports "CS user unknown" - when the user is plainly in the database. If I change user back to use Internal auth - it works a treat. If...