If/When you get a DNS problem or other network issue that causes the queue to fail it will fill up at a fair speed. Use the following to tell qmail to make another attempt at sending.
Or you can do this.
Run ps axl and find the process id (PID) for qmail-send
Run kill -s ALRM pid
To find out if there are any messages in the queue use
qmail-qstat or qmail-qread