well, this shouldn´t happen at all, but it happens if you empty the queue by just deleting the files in the /var/qmail/queue directory, anyhow, just recreate the file and you will be fine:
- /etc/init.d/qmail stop (or qmailctl stop)
- touch /var/qmail/queue/lock/sendmutex
- chown qmails:qmail /var/qmail/queue/lock/sendmutex
- /etc/init.d/qmail start (or qmailctl start)
that should do the trick!