通过aspx页面发送任何域名的电子邮件

Question

    string from = txtEmail.Text;

    using (MailMessage mm = new MailMessage(txtEmail.Text, "xyz@gmail.com"))
    {
        mm.Subject = "zaca";
        mm.Body = " vfsdfs ";
        mm.IsBodyHtml = false;
        SmtpClient smtp = new SmtpClient();
        smtp.Host = "smtp.gmail.com";
        smtp.EnableSsl = true;
        NetworkCredential NetworkCred = new NetworkCredential(txtEmail.Text, txtPassword.Text);
        smtp.UseDefaultCredentials = true;
        smtp.Credentials = NetworkCred;
        smtp.Port = 587;
        smtp.Send(mm);
        ClientScript.RegisterStartupScript(GetType(), "alert", "alert('Email sent.');", true);
    }

我正在使用此代码通过我的网页发送邮件。但它只能通过Gmail帐户向管理员发送邮件。我希望任何电子邮件服务的用户都可以通过我的网页发送邮件。请帮忙。