SMTP Service Extension for Client Identity

Multi-Factor Authentication has rapidly become a driving requirement for any internet based technology that requires authentication. While a large number of initiatives are active for providing solutions to this requirement for Web Browser based applications that can generally support real time human interaction for providing a secondary method of identification, legacy protocols such as SMTP authentication have not yet been revised to provide such support despite being a high-risk target for business email compromise, possibly as a result of authenticated SMTP activity generally expecting to be non-interactive in nature outside of Webmail logins. This document defines an extension to the SMTP service protocol called "CLIENTID" that a SMTP client can provide an additional unique identification token prior to standard credentials authentication that the server may then apply as an identify verification method in a similar manner to other Multi-Factor authentication techniques.