Exchange 2013 with Outlook 2010 asking sometime for credentials

Hi Everyone,

I'm running Exchange Server 2013 on a Windows Server 2012.

Some of my user are sometime asked for credentials.

It happens both at outlook launch and when outlook is running.

And when we just cancel the credentials windows without password, outlook is connected to exchange again.

Everyone is running Office 2010 with SP1.

I have tried to put Outlook anywhere to NTLM and Negotiation authentication but it doesn't work either.

Any idea ?

Thanks in advance