The Free On-line Dictionary of Computing (30 December 2018):

X.400

    The set of ITU-T communications standards
   covering electronic mail services provided by data networks.
   X.400 was widely used in Europe and Canada.

   X.400 addresses tend to be much longer than RFC 822 ones.
   They consist of a set of bindings for country (c),
   administrative domain (a), primary management domain (p),
   surname (s), given name (g).

   For example, the X.400 address,

   	c=gb;a=attmail;p=Universal Export;s=Bond;g=James;

   might be equivalent to RFC 822

   	James.Bond@UniversalExport.co.uk

   [Reference?]

   (2003-06-24)