A Mail User Agent (MUA), also referred to as an email client, is a computer application that allows you to send and retrieve email. A MUA is what you interact with, as opposed to an email server, which transports email. MUAs can be software applications, such as Outlook Express and Lotus notes, or they can be webmail services such as those provided by Yahoo!, Microsoft Outlook.com, and Gmail. 

MUAs are the component within the Simple Mail Transfer Protocol (SMTP) system responsible for creating email messages for transfer to a Mail Transfer Agent (MTA).