Barry, thanks heaps!!! The drive space warnings are gone...now I just need to get onto support to find out why this is happening. Thanks again for your help!
I had the same problem - lots of wool_xxxx files in /var/storage/chroot-smtp/tmp which were days old. I've raised this as an issue with Astaro support. A "find /var/storage/chroot-smtp/tmp/ -name "wool_*" -type f -mtime +2 -exec rm -f {} \;" run from cron is my workaround.