Cloud-based email became practical from about 2009, because:
- Workstation-cloud servers links had become sufficiently fast and cheap
- Vendors had produced versions of their software that could run on cloud servers, and service many different businesses concurrently
Nevertheless, businesses with more than 100 employees have been very slow to adopt cloud-based email. They have preferred to maintain the supporting technology themselves, installing software products such as MS Exchange and Lotus Notes on their own PCs and servers.
The  underlying arguments for cloud-based corporate email remain strong. Specifically, it provides a substantially lower total cost of ownership,  and enables the customer's IT staff to focus on more mission-critical tasks.
We predict that in 2016, businesses will start to embrace cloud-based email; and that in 2022 the majority of business email will be cloud-based.--David Ferris