What's the nature of your operation? If they're not giving you money then you're within your rights to say "sorry, don't use outlook"
Our customers are giving us lots of money. And the nature of the emails are account reminders, activation and internal message notifications.
Usually they are giving you money...
I ran a SaaS up until recently. Emailing users notifications for events was just a normal part of the app. Users got upset when the emails didn't come through. So then I had to be email tech support for something I had no control over. Lovely.