Email client primarily is a desktop or mobile application that enables users to receive and SEND emails directly on the desktop or mobile. Typically, email client requires an email address to be set up, mail server details & connectivity to the mail server to configure and use email service. These configuration and settings include email address, password, POP3/IMAP and SMTP address, port number, email aliases, and other related preferences.

A mail server or an email server is a server that supports email function in network and support clients to handles and delivers e-mail over a network. This can be over intranet or INTERNET. Email server receives emails from client computers and delivers them to other mail servers after proper authentication and authorization. Mail servers use MTA (Mail TRANSFER agent) with SMTP (Simple Mail Transfer Protocol) to support email transmission. You have used any open source free MTA or any paid version the BASIS of your requirements and security policies.