I didn't use logrotate for this. What I did instead was redirect the spool files to /dev/null to clean them out. I also stopped my email service before and started it after the cleanup. You can tailor this as you wish
echo "emailclean.sh starting on $(date)"
echo "Stopping fetchmail service"
#/sbin/initctl stop <service>
pkill fetchmail
sleep 10
echo "Cleaning up old mail"
cat /dev/null > /var/spool/mail/root
cat /dev/null > /var/spool/mail/webalert
sleep 10
echo "Starting fetchmail Service as user webalert"
sudo -u webalert /usr/bin/fetchmail
#/sbin/initctl start <service>
echo "Cleanup Complete! /var/spool/mail/root and webalert files sent to dev0"
echo "emailclean.sh finished cleanup on $(date)"