I make no claim to being a Groupwise integration expert, but I think you will be able to do what you want. There are a number of groupwise activex controls. http://developer.novell.com/products_dev/groupwise/ will give you information on these.

I have their controls on my machine but I used the Object API to do what I needed (find out who is in the person's address book and then pick out desired recipients and send messages) because after searching here on Groupwise I was able to come up with some examples of doing this. I have not worked on using the inbox, however. Anyway, as far as using the api there is full documentation available from Novell, although you'll have to go through an annoying registration routine to be able to download it. Hope this helps a little.