Email servers are responsible for sending, receiving, and storing messages. There are four main email servers: Gmail, Yahoo Mail, Outlook, and Zimbra. Gmail is a free web-based email service from Google. It offers users 15GB of storage and features such as labels, chat, and video calling. Yahoo Mail is a free web-based email service from Yahoo. It offers users 1TB of storage and features such as filters, folders, and address book. Outlook is a paid email service from Microsoft. It offers users 5GB of storage and features such as calendar, tasks, and contact management. Zimbra is an open source email server that can be installed on a variety of operating systems. It offers users unlimited storage and features such as collaboration tools and mobile access.