Got the message
qmail: alert: cannot start: unable to open mutex
after being a little too heavy handed clearing out a load of queued junk messages on one of my qmail servers. Had to do the following to get it working again:
touch /var/qmail/queue/lock/sendmutex
chown qmails:qmail /var/qmail/queue/lock/sendmutex
Thanks to http://ipucu.enderunix.org/view.php?id=708&lang=tr
