An email can be defined as an application works on either on personal computer or workstation and allows the end-user to send across, receive, and compose message. It is called a client because e-mail systems are built on client-server architecture, wherein a mail is sent through various clients to a central server, which again routes the sent mail to its intended endpoint. In other words, an email client, email reader, or more officially mail user agent (MUA), is a computer program used to access and administer the end-users email.
In addition, a web application that offers easy management of messages, configuration, and reception provisions are at times considered an email client, but most commonly it is called as web-mail.
There are many popular locally installed email clients, to name a few are Microsoft Outlook, IBM Lotus Notes, Pegasus Mail, Mozilla's Thunderbird, Eudora, KMail in the Kontact suite, Evolution and Apple Inc.'s Mail.
On the other hand, some of the most known web-based email clients that commonly used by the individuals and businesses are: Gmail, Lycos Mail, Mail.com, Outlook.com and Yahoo Mail.