ITU-T's/ISO's e-mail standard, from 1984 and onward. Titled "Message Handling System" (MHS).
This system is, of course, incompatible with SMTP, unless you use a mail gateway.
X.400's specification is more complicated than SMTP's. For example, X.400 mail is more secure (mail is moved only over trusted routes), it's more easily traceable, stuff like read receipts and receive receipts are there (you can get notification when the mail is received or read), and sender is certified.
Of course, it's standard. It's a complicated standard. No one in Internet wanted to implement this sort of E-mail stuff because reliability of this level was never a requirement; for example, why rely on trusted links when there's encryption? Some also see receipts as a privacy threat - and this is very critical in case of spam; Spammers wouldn't need to rely on "remove address" tricks to get lists of working addresses, all they'd need to do is to spam with receipts enabled. Of course, spamming wouldn't be profitable because X.400 carriers want money for each sent message and spammers want anonymity...
Anyway, I digress.
The major reason why X.400 was never "cool" was the particularly non-sexy addressing scheme. For example, why use a simple address like email@example.com (or, more modernly, Sten.Westerback@nokia.com) when you can do it the hard way: C=fi;o=nokia;ou=Nokia Telecom;g=Sten;s=Westerback... When I first came to Internet, my first question was this: "Okay, this 'Internet' thing looks cool, but tell me one thing: Is this the network that uses that ugly E-mail address scheme? ... Oh, that address type looks nice. I guess I'll sign up for Freenet Finland account then."
(This example was the only address I could think of at the time; taken from EUNet Finland E-mail addressing scheme change note, dated April 1, 1993.)
The mapping between X.400 and SMTP is described in RFC 987.
(Sources: Several web pages that each told very little but enough to refresh my memory. One page even marketed X.400 as an absolutely necessary companion for SMTP...)