While using Outlook 2010 the login box will pop back up after some time even after entering credentials.

The most common reason for that is autodiscover isn't configured correctly, or the autodiscover URL resolves to somewhere it shouldn't do. Issues with the autodiscover configuration in Exchange 2007 and higher are well documented. If you are using Exchange 2003, then it doesn't use autodiscover, so you should ensure that autodiscover.example.com (where example.com is the domain after the @ in your email address) doesn't resolve anywhere. Simon. Simon Butler, Exchange MVP Blog | Exchange Resources | In the UK? Hire Me.

If you have a mailbox that is hosted on a server that is running Microsoft Exchange Server 2003, this problem occurs if the Service Principal Names (exchangeAB/ExchangeServerName & exchangeAB/ExchangeServerName.example.com) are registered on the Exchange server and if the Exchange server is not a global catalog server. For details, see the KB927612. In addition, when the Windows Security window pop up, you can select the Remember my credentials option. Then check again.Noya Lau TechNet Community Support