An entity that offers services to the user as both a sender and a receiver of email messages. When preparing to send a mail message, the mail user agent allows the user to construct the message. The message is then passed by the user agent to the MTA (message transfer agent), which is responsible for passing the message to the recipient. At the point of delivery, the mail user agent allows the user to determine what messages await collection. The mail user agent may also be able to access a message store, which can act as a buffer for incoming mail.