What characters are allowed in an email address?

Question

I'm not asking about full email validation.

I just want to know what are allowed characters in user-name and server parts of email address. This may be oversimplified, maybe email adresses can take other forms, but I don't care. I'm asking about only this simple form: user-name@server (e.g. [e-mail]) and allowed characters in both parts.

Answer 1

Check for @ and . and then send an email for them to verify.

I still can't use my .name email address on 20% of the sites on the internet because someone screwed up their email validation, or because it predates the new addresses being valid.

Answer 2

As can be found in this Wikipedia link

The local-part of the email address may use any of these ASCII characters:

• uppercase and lowercase Latin letters A to Z and a to z;

• digits 0 to 9;

• special characters !#$%&'*+-/=?^_`{|}~;

• dot ., provided that it is not the first or last character unless quoted, and provided also that it does not appear consecutively unless quoted (e.g. [e-mail] is not allowed but "John..Doe"@example.com is allowed);

• space and "(),:;<>@[\] characters are allowed with restrictions (they are only allowed inside a quoted string, as described in the paragraph below, and in addition, a backslash or double-quote must be preceded by a backslash);

• comments are allowed with parentheses at either end of the local-part; e.g. john.smith(comment)@example.com and (comment)[e-mail] are both equivalent to [e-mail].

In addition to the above ASCII characters, international characters above U+007F, encoded as UTF-8, are permitted by RFC 6531, though mail systems may restrict which characters to use when assigning local-parts.

A quoted string may exist as a dot separated entity within the local-part, or it may exist when the outermost quotes are the outermost characters of the local-part (e.g., abc."defghi"[e-mail] or "abcdefghixyz"@example.com are allowed. Conversely, abc"defghi"[e-mail] is not; neither is abc\"def\"[e-mail]). Quoted strings and characters however, are not commonly used. RFC 5321 also warns that "a host that expects to receive mail SHOULD avoid defining mailboxes where the Local-part requires (or uses) the Quoted-string form".

The local-part postmaster is treated specially—it is case-insensitive, and should be forwarded to the domain email administrator. Technically all other local-parts are case-sensitive, therefore [e-mail] and [e-mail] specify different mailboxes; however, many organizations treat uppercase and lowercase letters as equivalent.

Despite the wide range of special characters which are technically valid; organisations, mail services, mail servers and mail clients in practice often do not accept all of them. For example, Windows Live Hotmail only allows creation of email addresses using alphanumerics, dot (.), underscore (_) and hyphen (-). Common advice is to avoid using some special characters to avoid the risk of rejected emails.

Answer 3

Wikipedia has a good article on this, and the official spec is here. From Wikipdia:

The local-part of the e-mail address may use any of these ASCII characters:

• Uppercase and lowercase English letters (a-z, A-Z)
• Digits 0 to 9
• Characters ! # $ % & ' * + - / = ? ^ _ ` { | } ~
• Character . (dot, period, full stop) provided that it is not the first or last character, and provided also that it does not appear two or more times consecutively.

Additionally, quoted-strings (ie: "John Doe"@example.com) are permitted, thus allowing characters that would otherwise be prohibited, however they do not appear in common practice. RFC 5321 also warns that "a host that expects to receive mail SHOULD avoid defining mailboxes where the Local-part requires (or uses) the Quoted-string form".

Answer 4

See RFC 5322: Internet Message Format and, to a lesser extent, RFC 5321: Simple Mail Transfer Protocol.

RFC 822 also covers email addresses, but it deals mostly with its structure:

 addr-spec   =  local-part "@" domain        ; global address     
 local-part  =  word *("." word)             ; uninterpreted
                                             ; case-preserved

 domain      =  sub-domain *("." sub-domain)     
 sub-domain  =  domain-ref / domain-literal     
 domain-ref  =  atom                         ; symbolic reference

And as usual, Wikipedia has a decent article on email addresses:

The local-part of the email address may use any of these ASCII characters:

• uppercase and lowercase Latin letters A to Z and a to z;
• digits 0 to 9;
• special characters !#$%&'*+-/=?^_`{|}~;
• dot ., provided that it is not the first or last character unless quoted, and provided also that it does not appear consecutively unless quoted (e.g. [e-mail] is not allowed but "John..Doe"@example.com is allowed);
• space and "(),:;<>@[\] characters are allowed with restrictions (they are only allowed inside a quoted string, as described in the paragraph below, and in addition, a backslash or double-quote must be preceded by a backslash);
• comments are allowed with parentheses at either end of the local-part; e.g. john.smith(comment)@example.com and (comment)[e-mail] are both equivalent to [e-mail].

In addition to ASCII characters, as of 2012 you can use international characters above U+007F, encoded as UTF-8.

For validation, see Using a regular expression to validate an email address.

The domain part is defined as follows:

The Internet standards (Request for Comments) for protocols mandate that component hostname labels may contain only the ASCII letters a through z (in a case-insensitive manner), the digits 0 through 9, and the hyphen (-). The original specification of hostnames in RFC 952, mandated that labels could not start with a digit or with a hyphen, and must not end with a hyphen. However, a subsequent specification (RFC 1123) permitted hostname labels to start with digits. No other symbols, punctuation characters, or blank spaces are permitted.

Answer 5

For simplicity's sake, I sanitize the submission by removing all text within double quotes and those associated surrounding double quotes before validation, putting the kibosh on email address submissions based on what is disallowed. Just because someone can have the John.."The*$hizzle*Bizzle"[e-mail] address doesn't mean I have to allow it in my system. We are living in the future where it maybe takes less time to get a free email address than to do a good job wiping your butt. And it isn't as if the email criteria are not plastered right next to the input saying what is and isn't allowed.

I also sanitize what is specifically not allowed by various RFCs after the quoted material is removed. The list of specifically disallowed characters and patterns seems to be a much shorter list to test for.

Disallowed:

    local part starts with a period ( [e-mail] )
    local part ends with a period   ( [e-mail] )
    two or more periods in series   ( [e-mail] )
    &’`*|/                          ( some&thing`[e-mail] )
    more than one @                 ( which@[e-mail] )
    :%                              ( mo:characters%mo:[e-mail] )

In the example given:

John.."The*$hizzle*Bizzle"[e-mail] --> [e-mail]

[e-mail] --> [e-mail]

Sending a confirm email message to the leftover result upon an attempt to add or change the email address is a good way to see if your code can handle the email address submitted. If the email passes validation after as many rounds of sanitization as needed, then fire off that confirmation. If a request comes back from the confirmation link, then the new email can be moved from the holding||temporary||purgatory status or storage to become a real, bonafide first-class stored email.

A notification of email address change failure or success can be sent to the old email address if you want to be considerate. Unconfirmed account setups might fall out of the system as failed attempts entirely after a reasonable amount of time.

I don't allow stinkhole emails on my system, maybe that is just throwing away money. But, 99.9% of the time people just do the right thing and have an email that doesn't push conformity limits to the brink utilizing edge case compatibility scenarios. Be careful of regex DDoS, this is a place where you can get into trouble. And this is related to the third thing I do, I put a limit on how long I am willing to process any one email. If it needs to slow down my machine to get validated-- it isn't getting past the my incoming data API endpoint logic.

Edit: This answer kept on getting dinged for being "bad", and maybe it deserved it. Maybe it is still bad, maybe not.

Answer 6

I created this regex according to RFC guidelines:

^[\\w\\.\\!_\\%#\\$\\&\\'=\\?\\*\\+\\-\\/\\^\\`\\{\\|\\}\\~]+@(?:\\w+\\.(?:\\w+\\-?)*)+$