queue getting full

Hi, my /var/qmail/queue/mess is getting full with messages like:

Received: (qmail 24122 invoked by uid 110); 9 Jul 2003 02:46:29 -0000
Received: from test@yahoo.com by pluto by uid 107 with qmail-scanner-1.12
(sweep: 2.10/3.58. . Clear:. Processed in 1.884876
X-Qmail-Scanner-Mail-From: test@yahoo.com via pluto
X-Qmail-Scanner: 1.12 (Clear:. Processed in 1.884876 secs)
Received: from unknown (HELO newserver) (218.242.81.172)
by 0 with SMTP; 9 Jul 2003 02:46:27 -0000
From: test@yahoo.com
To:
Subject: gRcZFRc0gz44gr42gbG3MlEWmLG0MLesO2KrNlCZOdKzgJeZ


Now qmail tryes to deliver this bounces - and of course without success, and
my postmaster mail account is getting full with bounce bounced messages.

Can anyone take some time and try to figure out how to avoid this? Is this
just spam?

tnx & br,
Greg