using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Net;
using System.Net.Mail;


namespace CO6009DissertationV5
{

    public partial class frmSendMsg : Form
    {

        public frmSendMsg()
        {
            InitializeComponent();
        }

        private void btnSend_Click(object sender, EventArgs e)
        {
            SmtpClient client = new SmtpClient();
            client.Host = "smtp.gmail.com";
            client.Port = 587;
            client.EnableSsl = true;

            System.Net.NetworkCredential userpassword = new System.Net.NetworkCredential();
            userpassword.UserName = "user@gmail.com";
            userpassword.Password = "password";

            client.Credentials = userpassword;

            MailMessage msg = new MailMessage("user@gmail.com", "user@gmail.com");
            **msg.To.Add(new MailAddress(txtBoxToEmail.Text));**
            msg.Body = "<b> Sender's Name: </b>" + txtBoxName.Text + "<p><b> Sender's E-mail: </b>" + txtBoxEmail.Text + "<p><b>Sender's Subject: </b>" + txtBoxSubject.Text + "<p><b>Sender's Message: </b>" + "<p>" + txtBoxMsg.Text;
            msg.Subject = txtBoxSubject.Text;
            msg.IsBodyHtml = true;

            try
            {
                client.Send(msg);
                lblMsgStatus.Text = "<p> Message Successfully Sent </p>";
            }

            catch (Exception ex)
            {
                lblMsgStatus.Text = "Send failed";
            }
        }
    }
}

msg.To.Add(new MailAddress(txtBoxToEmail.Text));

msg.To.Add(new MailAddress(txtBoxToEmail.Text));

private void btnSend_Click(object sender, EventArgs e)
{
    if (String.IsNullOrWhitespace(txtBoxToEmail.Text))
    {
        MessageBox.Show("To can't be empty!","Invalid Address",
                        MessageBoxButtons.OK,MessageBoxIcon.Error);
        return;
    }
...

private void textBox1_Validating(object sender,
                System.ComponentModel.CancelEventArgs e)
{
   string errorMsg;
   if(!ValidEmailAddress(textBox1.Text, out errorMsg))
   {
      // Cancel the event and select the text to be corrected by the user.
      e.Cancel = true;
      textBox1.Select(0, textBox1.Text.Length);

      // Set the ErrorProvider error with the text to display.
      this.errorProvider1.SetError(textBox1, errorMsg);
   }
}

private void textBox1_Validated(object sender, System.EventArgs e)
{
   // If all conditions have been met, clear the ErrorProvider of errors.
   errorProvider1.SetError(textBox1, "");
}
public bool ValidEmailAddress(string emailAddress, out string errorMessage)
{
   // Confirm that the e-mail address string is not empty.
   if(String.IsNullOrWhitespace(emailAddress.Length))
   {
      errorMessage = "e-mail address is required.";
         return false;
   }

   // Confirm that there is an "@" and a "." in the e-mail address, and in the correct order.
   if(emailAddress.IndexOf("@") > -1)
   {
      if(emailAddress.IndexOf(".", emailAddress.IndexOf("@") ) > emailAddress.IndexOf("@") )
      {
         errorMessage = "";
         return true;
      }
   }

   errorMessage = "e-mail address must be valid e-mail address format.n" +
      "For example 'someone@example.com' ";
      return false;
}

if (IsValidEmail(txtBoxToEmail.Text))
     {
            msg.To.Add(new MailAddress(txtBoxToEmail.Text));
     }

static bool IsValidEmail(string email)
    {
        try
        {
            var addr = new System.Net.Mail.MailAddress(email);
            return addr.Address == email;
        }
        catch
        {
            return false;
        }
    }