Actually, last time I looked, there were completely free MAPI connectors to break out of that ecosystem, as well as Free exchange clients that cover most, if not all functionality of the Outlook/Exchange combo, including calendaring.
It's been a while, so I don't remember the links anymore and I didn't take notes unfortunately, but they were easy to find (I believe, I started drilling down from sogo.nu and the enterprise gateway all in one live-cds like Zentyal, through which I found openchange). This was a bit from the other perspective (i.e. replace Exchange, not necessarily Outlook, but even that area benefitted from a huge improvement in relation to ~10-12 years ago, when I last was looking for that).