X.400
Originally published in 1984 by CCITT and later re-written in 1988 jointly by ISO and CCITT, X.400 is a standard that conforms to layer 7 of the OSI and is a standard used for transporting e-mail messages. X.400 is an alternative standard to the commonly used SMTP.
Example of SMTP e-mail address:
support@computerhope.com
Example of a X.400 e-mail address:
G=support; O=computerhope.com
Where G is the given name.
Additional information about the X.400 standard and its format can be found on RFC1327 and RFC1685.
Also see: CCITT, E-mail, ISO, OSI