If whitelisting is "application level" it's easier to diagnose with an "application (protocol) level" error. That is rather than a timeout or connection refuse

If whitelisting is "application level" it's easier to diagnose with an "application (protocol) level" error. That is rather than a timeout or connection refused, an smtp 550 or 530 error with exlenation (access/relay denied; client ip not in whitelist)?