If email is critical to your organisation, then don't outsource it to a public cloud provider. I don't really see why everyone is laughing/attacking the service. Its email, not life support.
If your business relies so heavily on email that potential uncontrollable downtime from a public cloud provider, don't migrate. Invest in your own system and sysadmins.
To the rest of us, where email being down doesn't mean the end of the world, we're quite happy with a public cloud provider.