Performing async operations that are guaranteed to execute within an IIS web site, even if an app pool is recycled

Certain actions that my web site's users carry out result in the sending of emails. The code that sends emails can block for a while, so I want to do this off their HTTP request handler's thread.

Currently I'm using something like:

ThreadPool.QueueUserWorkItem(o => {
    try
    {
        email.Send();
    }
    catch (Exception ex)
    {
        _log.Error("Error sending email", ex);
    }
});

For the most part, this works. However the web site runs in a hosted environment where the app pool can be recycled.

Every once in a while I don't receive an email that should have been sent, and I suspect that this work item on the threadpool's queue is being dropped during application pool recycling.

How can I perform an ansync operation like this and guarantee that it will complete in such a case?