Microsoft Office 2007 introduces a whole lot of new features - revamped UI, new file formats etc.  However, one thing is missing, which is support for the Outlook protocol (outlook://).

Developers wanting to open emails from other applications could use the Outlook protocol to get Microsoft Outlook to display them.  For example, outlook:00000000AEB50BC0928BD511A2700000E8D73A91C4A56101.

This is all well for Microsoft Outlook XP and 2003 (not quite sure about 2000); however, for Microsoft Outlook 2007 Microsoft have decided to no longer support it. 

Upgrading to Outlook 2007 will remove the Outlook protocol registry details which was setup by the previous version of Outlook.

Information about this is available [buried] in http://support.microsoft.com/kb/929590, which says:

Known issues when you develop custom solutions for Office Outlook 2007

The Outlook protocol no longer works outside Outlook

The Outlook protocol (Outlook://) has been changed. This protocol will only
work when you use it in Outlook. You can use this protocol on a folder home
page, on the Web toolbar, or in the body of an Outlook item.

Now registration of the Outlook protocol is straightforward, and results in outlook.exe being invoked with the /select command line argument with the value of the Outlook protocol following.

To reactivate the Outlook protocol with Outlook 2007 installed, simple copy the following into a .REG file and double click on it from Windows Explorer.

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\outlook]
“URL Protocol”=”"
@=”URL:Outlook Folders”

[HKEY_CLASSES_ROOT\outlook\DefaultIcon]
@=”C:\\Program Files\\Microsoft Office\\Office12\\OUTLOOK.EXE,-9403″

[HKEY_CLASSES_ROOT\outlook\shell]
@=”open”

[HKEY_CLASSES_ROOT\outlook\shell\open]
@=”"

[HKEY_CLASSES_ROOT\outlook\shell\open\command]
@=”\”C:\\Program Files\\Microsoft Office\\Office12\\OUTLOOK.EXE\” /select \”%1\”"

If Microsoft Outlook  2007 is not installed in default folder “C:\Program Files\Microsoft Office\Office12″ you will need to update the above accordingly.

Obviously this will only work whilst Office 2007 still supports the /select command line argument.

Also, in this day and age of automatic updates, Microsoft could, if they wished, revoke full support of the Outlook protocol at any time.

This workaround will give you some breathing space for an alternative solution to be developed.