is_email( string $email, bool $deprecated = false )

function is_email( $email, $deprecated = false ) {
	if ( ! empty( $deprecated ) )
		_deprecated_argument( __FUNCTION__, 'WP-3.0.0' );

	// Test for the minimum length the email can be
	if ( strlen( $email ) < 6 ) {
		/**
		 * Filters whether an email address is valid.
		 *
		 * This filter is evaluated under several different contexts, such as 'email_too_short',
		 * 'email_no_at', 'local_invalid_chars', 'domain_period_sequence', 'domain_period_limits',
		 * 'domain_no_periods', 'sub_hyphen_limits', 'sub_invalid_chars', or no specific context.
		 *
		 * @since WP-2.8.0
		 *
		 * @param bool   $is_email Whether the email address has passed the is_email() checks. Default false.
		 * @param string $email    The email address being checked.
		 * @param string $context  Context under which the email was tested.
		 */
		return apply_filters( 'is_email', false, $email, 'email_too_short' );
	}

	// Test for an @ character after the first position
	if ( strpos( $email, '@', 1 ) === false ) {
		/** This filter is documented in wp-includes/formatting.php */
		return apply_filters( 'is_email', false, $email, 'email_no_at' );
	}

	// Split out the local and domain parts
	list( $local, $domain ) = explode( '@', $email, 2 );

	// LOCAL PART
	// Test for invalid characters
	if ( !preg_match( '/^[a-zA-Z0-9!#$%&\'*+\/=?^_`{|}~\.-]+$/', $local ) ) {
		/** This filter is documented in wp-includes/formatting.php */
		return apply_filters( 'is_email', false, $email, 'local_invalid_chars' );
	}

	// DOMAIN PART
	// Test for sequences of periods
	if ( preg_match( '/\.{2,}/', $domain ) ) {
		/** This filter is documented in wp-includes/formatting.php */
		return apply_filters( 'is_email', false, $email, 'domain_period_sequence' );
	}

	// Test for leading and trailing periods and whitespace
	if ( trim( $domain, " \t\n\r\0\x0B." ) !== $domain ) {
		/** This filter is documented in wp-includes/formatting.php */
		return apply_filters( 'is_email', false, $email, 'domain_period_limits' );
	}

	// Split the domain into subs
	$subs = explode( '.', $domain );

	// Assume the domain will have at least two subs
	if ( 2 > count( $subs ) ) {
		/** This filter is documented in wp-includes/formatting.php */
		return apply_filters( 'is_email', false, $email, 'domain_no_periods' );
	}

	// Loop through each sub
	foreach ( $subs as $sub ) {
		// Test for leading and trailing hyphens and whitespace
		if ( trim( $sub, " \t\n\r\0\x0B-" ) !== $sub ) {
			/** This filter is documented in wp-includes/formatting.php */
			return apply_filters( 'is_email', false, $email, 'sub_hyphen_limits' );
		}

		// Test for invalid characters
		if ( !preg_match('/^[a-z0-9-]+$/i', $sub ) ) {
			/** This filter is documented in wp-includes/formatting.php */
			return apply_filters( 'is_email', false, $email, 'sub_invalid_chars' );
		}
	}

	// Congratulations your email made it!
	/** This filter is documented in wp-includes/formatting.php */
	return apply_filters( 'is_email', $email, $email, null );
}