Well... this answer is a bit too short, to investigate why the queue doesn't proceed on the next try - the log entry just points out, that there IS a queue, why the actual mail won't be delivered and why the new email is queued behind the others.
To tell postfix, to go to work immediatly you might want to force postfix with the command: "postqueue -f" ... but this doesn't solve the innitial problem, which you still have to investigate to get rid off.