Did you google for the given keywords / configuration parameters?
Then you'd have come across: http://www.postfix.org/postconf.5.html
The Postfix main.cf configuration file specifies a very small subset of all the parameters that control the operation of the Postfix mail system. Parameters not explicitly specified are left at their default values.
So this is the reason, both of these switches CAN be explicitly set in your main.cf. But if not set, then the default value is used.
According to the above documentation, I'd say changing smtpd_error_sleep_time back to 1 (default) won't help here.
Maybe it helps to see your main.cf and the exact error message from the logs. As for my understanding "bounce_queue_lifetime" (which is set to 5d by default) should work for this, as the message is kept for 5 days until it gets back to the original sender.