[Solved] ASP.Net SendAsync Error: An asynchronous module or handler completed while an asynchronous operation was still pending

Hi nauna,

You need to Cancel the Async operation.

Check with below code.

private bool mailSent { get; set; }
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        mailSent = false;
    }
}

private void SmtpClient_OnCompleted(object sender, AsyncCompletedEventArgs e)
{
    string token = (string)e.UserState;
    if (e.Error != null)
    {
        // Log Error.
        Response.Write(e.Error.Message);
    }
    else
    {
        mailSent = true;
    }
    mailSent = true;
}

protected void OnSendEmail(object sender, EventArgs e)
{
    StreamReader reader = new StreamReader(HttpContext.Current.Server.MapPath("~/Email/"));
    string readFile = reader.ReadToEnd();
    string myString = "";
    myString = readFile;
    myString = myString.Replace("{#abc}", "abc");

    MailMessage mail = new MailMessage();
    mail.From = new MailAddress(emailfrom, wsh.displayname);
    mail.To.Add(mm.emailID);
    mail.Bcc.Add(tcc);
    mail.Subject = "Email sending";
    mail.Body = myString.ToString();
    mail.IsBodyHtml = true;

    SmtpClient SmtpServer = new SmtpClient(smtp);
    SmtpServer.Port = port;
    SmtpServer.Credentials = new NetworkCredential(emailfrom, password);
    SmtpServer.EnableSsl = false;

    SmtpServer.SendCompleted += new SendCompletedEventHandler(SmtpClient_OnCompleted);
    string userState = "message";
    SmtpServer.SendAsync(mail, userState);

    if (!mailSent)
    {
        SmtpServer.SendAsyncCancel();
    }
    mail.Dispose();
    Response.Redirect("~/default");
}