Qmail is said to be secure and as such there are very less chances for spamming. In case of spamming, you can find out the culprit as said in the following example.
Try running /var/qmail/bin/qmail-qread. The utility on a server gave the following output.
20 Sep 2007 05:29:32 GMT #96945 24355 <email@example.com> bouncing
done remote firstname.lastname@example.org
done remote email@example.com
done remote firstname.lastname@example.org
done remote email@example.com
done remote firstname.lastname@example.org
done remote email@example.com
This shows that a lot of messages were sent from the address firstname.lastname@example.org and that all of them have bounced. In order to find the message content, to make sure it is spam, do the following.
# cd /var/qmail/queue/mess
# find . -iname 96945 ( 96945 is the message number which you get from the first line pasted above)
If the message content looks suspicious, you can take immediate action against the domain domain.com.