Update main.cf

Added the delay_warning_time (http://www.postfix.org/postconf.5.html#delay_warning_time) with 4 hours as setting. Postfix will inform the user that the e-mail has not been delivered, but that it will try for the next 5 days. 

There is also a setting called confirm_delay_cleared (http://www.postfix.org/postconf.5.html#confirm_delay_cleared), but according to the Postfix this can lead to a sudden burst of notifications at the end of a prolonged network outage.