# validar emails con dominios validos
function validateDnsEmail( $emailRaw )  {
	# Sacamos espacios y saltos
	$emailNr = eregi_replace( "\n", "", $emailRaw );
	$emailNr = eregi_replace( "\r", "", $emailRaw );
	$email = eregi_replace( " +", "", $emailNr );
	$email = strtolower( $email );
	# Lo pasamos por una regex para validar el formato
	if( ! eregi( "^[a-z0-9]+([_\\.-][a-z0-9]+)*". "@([a-z0-9]+([\.-][a-z0-9]+))*$", $email ) ){
		# Si no es valido....
		$feedback = 0;#'Error: "' . $email . '" is not a valid e-mail ';
		return $feedback;
	}else{
		#Si esta ok, sacamos el dominio
		$item = explode( "@", $email );
		$domain = $item["1"];
		if ( ( gethostbyname( $domain ) == $domain ) ){
			if ( gethostbyname( "www." . $domain ) == "www." . $domain ){
				$feedback = 0;#'Error: "' . $domain . '" is most probably not a valid domain!';
				return $feedback;
	 		}
			#Es Valido
			$feedback = 1;
			return $feedback;
		}else{
			#valido
			$feedback = 1;
			return $feedback;
		}
	}
}

Php validar email por dns beta 0.1

PHP