I think, you have two tasks here: sending message to users and receiving
message by users. The message could be in any format of your choice. Since
you want it to be received by Outlook and then make it an appoint ment in
Outlook, I'd think it is simplest to send it as email from the server
according to a receiver list. In this case, it is irrelevant how the message
is created (in VB.NET, Java, manually).
On the Outlook side, once user starts Outlook( you cannot force Outlook to
do something from remote machine without loggin user's consent), and emails
are received, you can have some code inside Outlook to check each received
email for a clue of whether the email is for the appointment. If yes, create
the appoint ment based on the content of the email and then send a
confirmation email back, if necessary.
The simplest way to do it in Outlook is to add some VBA code in
Outlook.Application.NewMail event handler.
Outlook security is a bit difficult to deal with whan you add some VBA code,
or COM add-in, or to automate it, it may confuse innocent users by
frightening security message pop-ups. That is why I suggested Visual Studio
2005 Tool for Office in pervious reply, since you post to a .NET NG.