User-agent support for email based HTML form submission, using a 'mailto' URL as the form action, was proposed in RFC 1867 section 5.6, during the HTML 3.2 era. Various web browsers implemented it by invoking a separate email program, using their own rudimentary SMTP capabilities, or by becoming Internet suites by implementing entire Email clients. Although sometimes unreliable, it was briefly popular as a simple way to transmit form data without involving a web server or CGI scripts.
