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Which is the current official standard that defines valid email addresses?

Wikipedia says there are two RFCs (5321 and 5322), but also an "extension" (RFC 6531), that (confusingly enough) is marked as "Proposed standard" (so it seems "unofficial", or at least not yet approved).

This site (which seems trustable), says that the official RFC is 5321.

Thanks in advance.

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friol
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  • possible duplicate of [What characters are allowed in email address?](http://stackoverflow.com/questions/2049502/what-characters-are-allowed-in-email-address) – Knut May 26 '14 at 21:16
  • The "extension" is for the very recent development of "internationalized" (i.e. non-ASCII) addresses, which explains it's non-final status. I believe the others are attempts to restate and collate much older standards and practices. There is also the pragmatic question of whether they do so correctly, in terms of matching the actual practice of current implementations. – IMSoP May 26 '14 at 21:25
  • "Trustable" and PHP? Not on the same day. – tripleee May 26 '14 at 21:36

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SMTP is 5321 and defines the transfer protocol. 5322 defines the data format of email -- what you are allowed to pass in as DATA in SMTP -- including the address format in the headers. The concept of a mailbox is introduced in 5321 but the detailed syntax is deferred to 5322.

Nominally, 6531 itself states that the extension does not modify or amend the base specification. It codifies a mechanism whereby compliant implementations can negotiate additional features by mutual agreement. In practice, though, it enables syntax which is outside the scope of the 5322 specification.

A "proposed standard" is a specification which is a candidate to become an official IETF standard, but many RFCs never get past this stage. That does not make them less official or trusted. See http://en.wikipedia.org/wiki/Internet_Standard for an overview of the process.

tripleee
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  • Thanks. And RFC 6531? Is it part of the spec? – friol May 26 '14 at 21:20
  • I think the concept that RFC 6531 doesn't modify previous standards is a rather technical one. Rather than defining a mechanism for extending, I would rather say it *makes use of such a mechanism*. If you want to interact with servers implementing that extension, your definition of a valid email address does indeed need to be widened (unlike internationalized domain names, there is no ASCII-encoded form of an internationalized address, even as a fallback, just an accept/reject mechanism). So in practice, there is a modified standard of "SMTP with SMTPUTF8 extensions enabled". – IMSoP May 26 '14 at 21:39
  • Thanks for your comment. The fact that RFC6531 uses the ESMTP extension mechanism to define an extension I think is unimportant here. But I basically agree about your main poinṭ. I'll try to rephrase the answer. Thanks! – tripleee May 27 '14 at 05:37