Hi

1) Exchange Client extensions work with all versions of Outlook and you can
distribute them as msi in group policies assigned to machines.
The problem is the "archived documentation". Here is an old MSDN article:
http://msdn.microsoft.com/archive/default.asp?url=/archive/en-us/dnarout98/html/
msdn_outextend.asp
You can also use the old MSDN 2001 for Visual Studio 6 to get more details.

2) A COM-Addins have the advantage of being documented in the current MSDN
but in Outlook they are a bit more painfull as commandbars and their buttons
have to be added to all opened windows dynamically, not only once when the
addin is created. Outlook speaks of Explorers (the Outlook main window) and
Inspectors (windows to visualize items like mail, contacts...). There are
collection objects for both and you need to advise the
NewExplorer/NewInspector evenst and add your button to the new
explorer/inspector.
This is a bit more complex but you get better support for different button
types.
This can also be distributed as MSI in a group policy. You can choose user
or machine distribution.

3) VSTO. This is a .Net wrapper around COM-Addins so you can use C# or
VB.Net. I didn't go that route as you have to target a single Office
version. There are unsupported ways of doing Office 2003/2007 in one
assembly but I would want to go that way.

So I recommend 2)

--
SvenC

If you want a toolbar button, the best way would be to use an add-in. See http://www.outlookcode.com/article.aspx?id=56