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.

data/conf/postfix/main.cf

@@ -20,6 +20,7 @@ broken_sasl_auth_clients = yes
 disable_vrfy_command = yes
 maximal_backoff_time = 1800s
 maximal_queue_lifetime = 5d
+delay_warning_time = 4h
 message_size_limit = 104857600
 milter_default_action = accept
 milter_protocol = 6