random users getting prompted for credentials in outlook 2010

i i have random users getting prompted for credentials after having outlook opened for a while.

i checked these articles:
http://social.technet.microsoft.com/Forums/en-US/exchangesvradmin/thread/7256b339-2a73-44d5-9758-319d33abe02c
http://social.technet.microsoft.com/Forums/en-US/outlook/thread/6cfd254e-3e7f-4528-9c53-f9d429ad5c3f

and cant seem to find what iis security settings i need to check.

i noticed that if i changed the user to NTLM for logon network security, it works fine.

but i dont need to do that for everyone.

any ideas?