Default queue life time is 7 days. That means, your mails will be stored in queue for 7 days if it not able to deliver.
We can reduce this queue time with following steps.
To check current queue life time:
/var/qmail/bin/qmail-showctl | grep life
Time mentioned as seconds,
To set queue life time:
echo "172800" > /var/qmail/control/queuelifetime
Here I've set queuelifetime is 2 days.
For changes to take effect
service qmail restart
or
/etc/init.d/qmail restart