Email Validation is a tough subject to nail down, as their are tons of available code snippets that claim to validate an email address.  To make it easier below you will find the requirements for a valid email address as outlined by RFC 2822 followed by a script that can be used to validate email addresses provided by linuxjournal.com.

Email Address Requirements

1. An e-mail address consists of local part and domain separated by an at sign (@) character (RFC 2822 3.4.1).
2. The local part may consist of alphabetic and numeric characters, and the following characters: !, #, $, %, &, ‘, *, +, -, /, =, ?, ^, _, `, {, |, } and ~, possibly with dot separators (.), inside, but not at the start, end or next to another dot separator (RFC 2822 3.2.4).
3. The local part may consist of a quoted string—that is, anything within quotes (“), including spaces (RFC 2822 3.2.5).
4. Quoted pairs (such as @) are valid components of a local part, though an obsolete form from RFC 822 (RFC 2822 4.4).
5. The maximum length of a local part is 64 characters (RFC 2821 4.5.3.1).
6. A domain consists of labels separated by dot separators (RFC1035 2.3.1).
7. Domain labels start with an alphabetic character followed by zero or more alphabetic characters, numeric characters or the hyphen (-), ending with an alphabetic or numeric character (RFC 1035 2.3.1).
8. The maximum length of a label is 63 characters (RFC 1035 2.3.1).
9. The maximum length of a domain is 255 characters (RFC 2821 4.5.3.1).
10. The domain must be fully qualified and resolvable to a type A or type MX DNS address record (RFC 2821 3.6).

Validation Script

function validEmail($email) {
    $isValid = true;
    $atIndex = strrpos($email, "@");
    if (is_bool($atIndex) && !$atIndex) {
        $isValid = false;
    } else {
        $domain = substr($email, $atIndex+1);
        $local = substr($email, 0, $atIndex);
        $localLen = strlen($local);
        $domainLen = strlen($domain);
        if ($localLen < 1 || $localLen > 64) {
            // local part length exceeded
            $isValid = false;
        } else if ($domainLen < 1 || $domainLen > 255) {
            // domain part length exceeded
            $isValid = false;
        } else if ($local[0] == '.' || $local[$localLen-1] == '.') {
            // local part starts or ends with '.'
            $isValid = false;
        } else if (preg_match('/\.\./', $local)) {
            // local part has two consecutive dots
            $isValid = false;
        } else if (!preg_match('/^[A-Za-z0-9\-\.]+$/', $domain)) {
            // character not valid in domain part
           $isValid = false;
        } else if (preg_match('/\.\./', $domain)) {
            // domain part has two consecutive dots
            $isValid = false;
        } else if (!preg_match('/^(\\.|[A-Za-z0-9!#%&`_=\/$'*+?^{}|~.-])+$/', str_replace("\\","",$local))) {
            // character not valid in local part unless
            // local part is quoted
            if (!preg_match('/^"(\\"|[^"])+"$/', str_replace("\\","",$local))) {
               $isValid = false;
            }
        }
        if ($isValid && !(checkdnsrr($domain,"MX") || checkdnsrr($domain,"A"))) {
            // domain not found in DNS
            $isValid = false;
        }
    }
    return $isValid;
}