try
            {
                MailMessage mail = new MailMessage();
                SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");

                mail.From = new MailAddress("myUserName@gmail.com");
                mail.To.Add("myDestination@yahoo.com");
                mail.Subject = "Test Mail";
                mail.Body = "This is for testing SMTP mail from GMAIL";

                SmtpServer.Port = 465;// 587;
                SmtpServer.EnableSsl = true;
                SmtpServer.Credentials = new System.Net.NetworkCredential("myUserName", "myPasword");
                SmtpServer.EnableSsl = true;

                SmtpServer.Send(mail);
                MessageBox.Show("mail Send");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.ToString());
            }

        private void button2_Click(object sender, EventArgs e)
        {//https://www.mimekit.net/docs/html/Creating-Messages.htm
            var message = new MimeMessage();
            message.From.Add(new MailboxAddress("Herm", "jodahush@gmail.com"));
            message.To.Add(new MailboxAddress("mann", "jodahush@yahoo.com"));
            message.Subject = "How you doin?";

            message.Body = new TextPart("plain")
            {
                Text = @"Hey Alice,

                What are you up to this weekend? Monica is throwing one of her parties on
                Saturday and I was hoping you could make it.

                Will you be my +1?

                -- Joey
                "
            };

            
        }

using MailKit;
using MimeKit;
using MailKit.Net.Smtp;
using MailKit.Security;

private void button1_Click(object sender, EventArgs e)
        {
            var message = new MimeMessage();
            message.From.Add(new MailboxAddress("Joey", "xxx@gmail.com"));
            message.To.Add(new MailboxAddress("Alice", "xxx@yahoo.com"));
            message.Subject = "How you doin?";

            message.Body = new TextPart("plain")
            {
                Text = @"Hey Alice,

                    What are you up to this weekend? Monica is throwing one of her parties on
                    Saturday and I was hoping you could make it.

                    Will you be my +1?

                    -- Joey
"
            };

            using (var client = new SmtpClient())
            {
                client.Connect("smtp.gmail.com", 465, SecureSocketOptions.SslOnConnect);

                client.Authenticate("xxx", "xxx");

                client.Send(message);



                client.Disconnect(true);
        }

            using (var client = new SmtpClient())
            {
                //client.Connect("smtp.gmail.com", 465, SecureSocketOptions.SslOnConnect);
                client.Connect("smtp.gmail.com", 587, SecureSocketOptions.StartTlsWhenAvailable);

                client.Authenticate("xxx", "ccc");

                client.Send(message);

  System.Diagnostics.Process.Start("mailto:hush4@web.de?subject=test&body=teesttext");