SSO from MS sharepoint portal to SAP EP

Dear,

I have configured the SPNego on J2EE, and if I logon to the desktop of the domain and then navigate to SAP EP, I can logon to EP without entering user and password and kerbtray displays the Kerberos ticket. But if I logon to MS sharepoint from a desktop which is not in the domain and then navigate to SAP EP, I still need to enter the user and password. How can I fulfil the SSO from sharepoint to EP?

Thanks.