Strange Outlook issue.

We have a couple of users being constantly prompted for Outlook credentials. I have checked that the option to ask for these is turned off and also went as far as rebuilding profiles. I was building a laptop for one of the users and it was giving me this issue and even rebuilt the laptop to see if this would help. Wee bit stuck with this one! All credentials under cached credentials have been cleared (this is proven with the rebuild of the laptop for that particular user)