Gmail is used by over 1 billion users every single day. Gmail provides your users with the same, familiar interface that they are accustomed to using day in and day out, whether at home, at work, or on the go.
Gmail includes a custom email for your domain, with scalability so you can easily add new users as you expand. You can choose how much storage you have. There are 3 sizes Gmail has to offer: 15GB, 30GB and Unlimited, although most popular choice is the unlimited option.