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I know there are lots of RFCs that changed how an email can be made. However, most of the RFCs don't apply to for example outlook.

First of all, what are RFCs exactly? And why don't for example outlook has all RFCs applied?

For example, if I try to make a mail at outlook called ____.__.____@outlook.com, it will give me an error it has to start with a letter. HOWEVER:

According to isemail.info, it tells me the email is valid.

The same goes for example___~@outlook.com, it gives an error, but on isemail.info it says

The email address tested was example___~@test.com

The general result is: Address is valid

The specific diagnosis is: Address is valid. 
Please note that this does not mean the address actually exists, nor even that the domain actually exists. 
This address could be issued by the domain owner without breaking the rules of any RFCs.

Now, my second question is: if I would for example buy a VPS and setup an email server, how would I make emails like ~_this.is.example.email_~@example.com possible?

My last question is about the second part (after the @).

According to isemail.info, this email is valid: example@~example.com

The email address tested was example@~example.com

The general result is: The address is only valid according to the broad definition of RFC 5322. It is otherwise invalid.

The specific diagnosis is: Address is RFC 5322 compliant but contains domain characters that are not allowed by DNS

However, I never have seen a domain like ~example.com. I assume it'd be only for local usage, but I'm not entirely sure as I've got barely any server and this kind of stuff knowledge.

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You have to read everything your isemail.info states:

The general result is: The address is only valid according to the broad definition of RFC 5322. It is otherwise invalid.

The specific diagnosis is: Address is RFC 5322 compliant but contains domain characters that are not allowed by DNS

So in theory, this address is OK (email RFC-wise), but as DNS doesn't accept (all) special characters, it can't be used via the internet.

Every email client should implement the basic RFCs, like line-length, header data and so on, but it doesn't have to do so. It's completely fine to create a mail agent which doesn't conform to any RFCs relating to email, but don't expect the emails to be delivered by MTAs / MDAs / anything, really.

As for your first question "what is a RFC", google:

https://www.ietf.org/rfc.html

https://en.wikipedia.org/wiki/Request_for_Comments

A Request for Comments (RFC) is a type of publication from the Internet Engineering Task Force (IETF) and the Internet Society (ISOC), the principal technical development and standards-setting bodies for the Internet.

An RFC is authored by engineers and computer scientists in the form of a memorandum describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems. It is submitted either for peer review or simply to convey new concepts, information, or (occasionally) engineering humor. The IETF adopts some of the proposals published as RFCs as Internet Standards.

Request for Comments documents were invented by Steve Crocker in 1969 to help record unofficial notes on the development of ARPANET. RFCs have since become official documents of Internet specifications, communications protocols, procedures, and events.

EDIT for clarification:

The RFC 2181 states which special characters can or should be used. Again: in your very own private network, even your DNS can resolve anything you want, in fact, you can do anything you want. But don't let it get out into the wild (internet), you'll run into serious trouble.

  • Ah, so from what I understand most of the RFCs that has to do with the second part after the @ can only be used in a local network but not outside that? And what I kind of don't understand is the thing about the first part. The email address before the @ hasn't got anything to do with DNS. So I don't understand why `_some~example@outlook.com doesn't work. And it should work if you setup a mail server on a VPS right? – Joshua Bakker Apr 4 '17 at 14:28
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    @JoshuaBakker because outlook.com doesn't want it to work. They can implement any restrictions they want. In your own mailserver, you too are free to do what you want. I'll edit my answer. – Lenniey Apr 4 '17 at 14:34

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