olemus
elektronposti ITU standardite sari, mis põhineb OSI mudelil ning on SMTP alternatiiv sõjalistes jt erirakendustes

ülevaateid
https://www.techopedia.com/definition/5792/x400
https://en.wikipedia.org/wiki/X.400
http://pubs.opengroup.org/onlinepubs/9693999099/toc.pdf
https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-F.400-199906-I!!PDF-E&type=items
https://ecosio.com/en/blog/what-is-x400/

turvalisus
https://www.kb.cert.org/vuls/id/927278
https://www.rfc-editor.org/rfc/rfc3854