I started getting some munin notifications about disk usage and found out my root partition was getting full so I started to investigate.
First to find out the culprit.
$ sudo du -h --max-depth=2 /var |less
This reported that my /var/postfix/spool contained 4Gb of data so I needed to empty the queue.
Enter the snippet
for i in `mailq|grep '@' |awk {'print $1'}|grep -v '@'`; do postsuper -d $i ; done