>Think my last post was correct. Maybe remove the using when creating the SmtpClient and put 'client.Dispose()' in the delegate callback. i.e in outline:

SmtpClient client = new SmtpClient();
>                    {
>                        client.SendCompleted += new SendCompletedEventHandler(SendCompletedCallback);
>                            try
>                            {
>                                 client.SendCompleted += delegate
>                                {
>                                  client.Dispose();
>                                    lock (numRemainingLock)
>                                    {
>                                        if (--numRemaining == 0)
>                                        {
>                                            waitHandle.Set();
>                                        }
>                                    }
>                                };
>                                client.SendAsync(msg, userState);                 // Send our email.   
>                            }
>                            catch (Exception)
>                            {
>                                msg.Dispose();
>                                // throw;
>                            }
>                        }
>                    }

Superb! Thanks once again Viv. What is interesting is that all the email is now being sent, but the code never hits the SendCompletedCallback.

Oh, it also does not seem to be running asynchronously as I don't get control back in the UI for quite a while.

One of these days I'll understand C# :(