Implementation of SendMailAsync that correctly cancels the operation when CacellationToken is cancelled.

SendMailAsync.cs

public async Task SendMailAsync(MailMessage message, CancellationToken ct)  
{
    try
    {
        var task = _smtpClient.SendMailAsync(message);
        using (ct.Register(_smtpClient.SendAsyncCancel))
        {
            try
            {
                await task;
            }
            catch (OperationCanceledException exception)
            {
                if (exception.CancellationToken == ct)
                {
                    Trace.TraceWarning("Operation has been canceled.");
                    return;
                }

                throw;
            }
        }
    }
    catch (Exception exception)
    {
        Trace.TraceError("Unexpected exception occured; Exception details: {0}", exception.ToString());
    }

}