When publishing Exchange 2007 with multiple web sites (under IIS) you may end up with the external published URL doesn’t authenticate well. In my case I had ISA 2006 publishing the Exchange 2007 SP1, and suddenly the OWA stopped working and I got the URL https://mail.domain.com/owa/auth/owaauth.dll and the error message was (as usual) totaly unusable saying the page cannot be displayed, blah, blah, blah.
However, looking into IIS logs, and checking here and there, do some search on the internet finally figured out it might be an issue with the IIS itself with that dll file.
Just went to the IIS manager, right-click on the “Default Web Site” and select properties.
Under the “ISAPI Filters” I checked the Exchange OWA Cookie….” filter, and clicked Edit
Copy and paste both values into a text file and make sure I have it saved on the disk.
Removed the filter, restarted the IIS, then add it back using the same values and restart the IIS again.
IT WORKS :)