Re: Microsoft 365?
"There have been some pretty major outages of Office365 in the US, several of them over the past few years. The problem is that Exchange was never designed to be web scale."
Exchange has been designed for hosting / multi-tenant deployments for about a decade now. It's a scale out model so there really are no practical limits on scalability as long as you can throw additional VMs at it when needed... Current recommended per instance limits are 192GB RAM and 24 processor cores. (Microsoft have said that the supported core count will be upped in the future.)