Annoying login box popping up after starting Outlook, this keeps popping up throughout my working day. 

This solved the problem:

IIS Mgr on SBS 2008 / - / SBS Web Applications / - SSL Settings / Client certificates - Stop / Start web server after changing settings: 

Autodiscover: Ignore 
EWS: Ignore 
OAB: Accept *) 
OWA: Accept *) 
Rpc:  Accept *) 
RpcWithCert: Accept *)