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 <firstname.lastname@example.org> bouncing
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
done remote firstname.lastname@example.org
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