With the advancement of faster and faster internet connections, moving certain data to the cloud becomes more feasible.
The advantages of cloud based solutions are that you do not have to maintain the same level of hardware locally and can often rely on the cloud provider to keep your data backed up.
Not everything will be suitable for cloud hosting, for example unless you can afford an expensive high bandwidth leased line, uploading very large files frequently may prove unrealistic.
Cost is another consideration, running your own on-premises mail server can be much cheaper if you have many users, but for smaller companies the cloud can be a good fit for email services