Varun Gupta developed a mail server using PHP, HTML5, CSS3 and JavaScript. The mail server requires a PC with 20GB hard disk, 256MB RAM, Pentium 1 or higher processor, XAMPP server, a web browser like Chrome or Firefox, MySQL database, design tools like Dreamweaver or Notepad++, and the PHP language. The mail server can receive and deliver emails between client computers and other mail servers, with an emphasis on single-user mail being located on a single server to enhance reliability and access.