client.Credentials = new System.Net.NetworkCredential("my@gmail.com", "myGmailpassword");

client.Send(msg);

protected void Button1_Click(object sender, EventArgs e)
    {
        string str = SendMail("yourid@gmail.com", "receiver@domain.com", "subject", "body");
        Response.Write(str);
    }

    public string SendMail(string from, string to, string subject, string body)
    {

        // Building the Message
        MailMessage msg = new MailMessage();
        msg.To.Add(to);
        msg.From = new MailAddress(from, "Name", System.Text.Encoding.UTF8);
        msg.Subject = subject;
        msg.SubjectEncoding = System.Text.Encoding.UTF8;
        msg.Body = body;
        msg.BodyEncoding = System.Text.Encoding.UTF8;
        msg.IsBodyHtml = true;
        msg.Priority = MailPriority.High;

        // Smtp configuration
        SmtpClient client = new SmtpClient();
        client.Credentials = new System.Net.NetworkCredential("yourid@gmail.com", "yourpassword");
        client.Port = 25; //25 or use 587 or 465
        client.Host = "smtp.gmail.com";
        client.EnableSsl = true;
        try
        {
            client.Send(msg);
            return "Send Mail Successfully";
        }
        catch (Exception ex)
        {
            return ex.Message;
        }
    }