Outlook Anywhere with Basic auth.

We use Exchange 2010 with Outlook 2010 and our clients are domain joined.
Unfortunately we have to use Basic authentication for Outlook Anywhere, we can't use NTLM.
I know for Basic Authentication the user get's prompt for password.
But how can we avoid these login prompts , I always thought if you add the Outlook Anywhere url to IE Local Intranet Zone sites the user wasn't prompted for password, or is this incorrent?