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copied!<p>See <a href="http://tools.ietf.org/html/rfc5322" rel="noreferrer">RFC 5322: Internet Message Format</a> and, to a lesser extent, <a href="http://tools.ietf.org/html/rfc5321" rel="noreferrer">RFC 5321: Simple Mail Transfer Protocol</a>.</p> <p><a href="http://tools.ietf.org/html/rfc822#section-6.1" rel="noreferrer">RFC 822</a> also covers email addresses, but it deals mostly with its structure:</p> <pre><code> 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 </code></pre> <p>And as usual, Wikipedia has a decent <a href="https://en.wikipedia.org/wiki/Email_address#Local-part" rel="noreferrer">article on email addresses</a>:</p> <blockquote> <p>The local-part of the email address may use any of these ASCII characters:</p> <ul> <li>uppercase and lowercase Latin letters <code>A</code> to <code>Z</code> and <code>a</code> to <code>z</code>;</li> <li>digits <code>0</code> to <code>9</code>;</li> <li>special characters <code>!#$%&'*+-/=?^_`{|}~</code>;</li> <li>dot <code>.</code>, 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. <code>John..Doe@example.com</code> is not allowed but <code>"John..Doe"@example.com</code> is allowed);</li> <li>space and <code>"(),:;<>@[\]</code> 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);</li> <li>comments are allowed with parentheses at either end of the local-part; e.g. <code>john.smith(comment)@example.com</code> and <code>(comment)john.smith@example.com</code> are both equivalent to <code>john.smith@example.com</code>.</li> </ul> </blockquote> <p>In addition to ASCII characters, <a href="https://tools.ietf.org/html/rfc6531" rel="noreferrer">as of 2012</a> you can use international <a href="http://www.utf8-chartable.de/unicode-utf8-table.pl" rel="noreferrer">characters above</a> <code>U+007F</code>, encoded <a href="https://en.wikipedia.org/wiki/UTF-8#Description" rel="noreferrer">as UTF-8</a>.</p> <p>For validation, see <a href="https://stackoverflow.com/questions/201323/using-a-regular-expression-to-validate-an-email-address">Using a regular expression to validate an email address</a>.</p> <p>The <code>domain</code> part is defined <a href="https://en.wikipedia.org/wiki/Hostname#Restrictions_on_valid_hostnames" rel="noreferrer">as follows</a>:</p> <blockquote> <p>The Internet standards (Request for Comments) for protocols mandate that component hostname labels may contain only the ASCII letters <code>a</code> through <code>z</code> (in a case-insensitive manner), the digits <code>0</code> through <code>9</code>, and the hyphen (<code>-</code>). The original specification of hostnames in <a href="http://tools.ietf.org/html/rfc952" rel="noreferrer">RFC 952</a>, mandated that labels could not start with a digit or with a hyphen, and must not end with a hyphen. However, a subsequent specification (<a href="http://tools.ietf.org/html/rfc1123" rel="noreferrer">RFC 1123</a>) permitted hostname labels to start with digits. No other symbols, punctuation characters, or blank spaces are permitted.</p> </blockquote>

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