System.Security.SecurityException: Request for the permission of type 'System.Net.Mail.SmtpPermission' failed

Hello friends, I am working on my code with a method that sends mail from a Gmail account. This method was published here in aspsnippets.com, and I use it as a reference.

Error Message: System.Security.SecurityException: Request for the permission of type 'System.Net.Mail.SmtpPermission' failed.

but it throws the following error, and when I have running on my hosting server.

this is the code

using (MailMessage mm = new MailMessage("sender@gmail.com", txtEmail.Text))
    {
        mm.Subject = "Account Activation";
        string body = "Hello " + txtUsername.Text.Trim() + ",";
        body += "<br /><br />Please click the following link to activate your account";
        body += "<br /><a href = '" + Request.Url.AbsoluteUri.Replace("CS.aspx", "CS_Activation.aspx?ActivationCode=" + activationCode) + "'>Click here to activate your account.</a>";
        body += "<br /><br />Thanks";
        mm.Body = body;
        mm.IsBodyHtml = true;
        SmtpClient smtp = new SmtpClient();
        smtp.Host = "smtp.gmail.com";
        smtp.EnableSsl = true;
        NetworkCredential NetworkCred = new NetworkCredential("sender@gmail.com", "<password>");
        smtp.UseDefaultCredentials = true;
        smtp.Credentials = NetworkCred;
        smtp.Port = 587;
        smtp.Send(mm);
    }

this is my error

Thank you for your help. Thank you, Regards.