..mailbox unavailable server..... server response 5.7.3  Requested action aborted; user not authenticated.

private void gmailTogmail(object sender, EventArgs e)
        {
            var mail = new MailMessage();
            var c = new MailAddressCollection { txtto.Text.Trim() };
            mail.From = new MailAddress(txtfrom.Text.ToLower().Trim());

            // mail.To.Add((c[0].Address));
            mail.To.Add(txtto.Text.ToLower());
            mail.Subject = "for hilk";
            mail.Body = "some heeeeeee";
            mail.IsBodyHtml = true;
            var smtp = new SmtpClient
            {
                Host = "smtp.gmail.com",
                Port = 587,
                Credentials = new NetworkCredential(txtfrom.Text.Trim(), txtpass.Text.Trim()),
                EnableSsl = true
            };

            smtp.Send(mail);
            MessageBox.Show("ok");

        }

        private void gmailToHotmail(object sender, EventArgs e)
        {
            SmtpClient SmtpServer = new SmtpClient("smtp.live.com");
            var mail = new MailMessage();
            mail.From = new MailAddress(txtfrom.Text.Trim());
            mail.To.Add(txtto.Text.Trim());
            mail.Subject = "Your Sub";
            mail.IsBodyHtml = true;
             SmtpServer.Port = 587;
            SmtpServer.Timeout = 5000;
            SmtpServer.UseDefaultCredentials = false;
            SmtpServer.Credentials = new System.Net.NetworkCredential(txtfrom.Text.Trim(),txtpass.Text.Trim());
            SmtpServer.EnableSsl = true;
            SmtpServer.Send(mail);
            MessageBox.Show("ok");
        }