Bus dev/sales can survive a day without email every now and again (in my experience, far more than a day without a phone system). Everything will resume the next day, sure it pays the bills but a six hour outage is not "mission critical", it wont stop the "check" from arriving (email is not a payment method after all).

I have known large (email dependent) businesses have 2 day outages on the traditional Microsoft mail stack too. There are coping strategies. It is annoying not mission critical.