A mail server (or email server) is a computer system that sends and receives email.In many cases, web servers and mail servers are combined in a single machine.However, large ISPs and public email services (such as Gmail and Hotmail) may use dedicated hardware for sending and receiving email. In order for a computer system to function as a mail server, it must include mail server software This software allows the system administrator to create and manage email accounts for any domains hosted on the server