Somehow Outlook 2007 kept asking for a password for the external domain name.
I’ve found a solution on the following web-page:
Source: http://social.technet.microsoft.com/Forums/…..
In short: quote ‘RickMac2001’:
“Using SBS2008 with the integral Exchange 2007. All my clients, whether inside my firewall or outside, kept prompting for creds. Your solution seems to have solved my issues. All I did was go into my IIS, change the OAB and Autodiscover sites to accept credentials in SSL, and “Voila!”, it seems to start working again.”