Is this safe?

gistfile1.cs

public class Mailbox
{
    private ConcurrentQueue<Envelope> queue = new ConcurrentQueue<Envelope>();

    private int scheduled = 0;
    private volatile bool hasUnScheduledMessages = false;
    private WaitCallback run;

    public Mailbox ()
    {
        run = new WaitCallback(Run);
    }

    public void Run(object state)
    {
        Envelope tmp;
        while (queue.TryDequeue(out tmp))
        {
            //code to process each message
        }

        scheduled = 0;
        if (hasUnScheduledMessages)
        {
            hasUnScheduledMessages = false;
            Schedule();
        }
    }

    public void Post(Envelope m)
    {
        hasUnScheduledMessages = true;
        queue.Enqueue(m);
        Schedule();
    }

    private void Schedule()
    {
        //only schedule if we idle
        if (Interlocked.Exchange(ref scheduled, 1) == 0)
        {
            ThreadPool.QueueUserWorkItem(run);
        }
    }
}