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    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        public static void Sende_EMail(string emailUser, string emailAbsender, string smtpHost, string smtpUser, string smtpPasswort, string betreff, string bodyText)
        {
            //E-Mail mit Authentifizierung, SmtpClient
            MailMessage mail = new MailMessage();
            MailAddress from = new MailAddress(emailAbsender);
            mail.To.Add(emailUser);
            mail.From = from;
            mail.Subject = betreff;
            mail.Body = bodyText;
            string host = smtpHost;
            int port = 25;
            SmtpClient client = new SmtpClient(host, port);

            System.Net.NetworkCredential nc = new NetworkCredential(smtpUser, smtpPasswort);
            client.Credentials = nc;

            client.Send(mail); //Hier tritt der Fehler auf
        } 

        private void button1_Click(object sender, EventArgs e)
        {
            Sende_EMail("<name>@vtg.admin.ch", "<name>@swissonline.ch", "smtp.hispeed.ch", "<name>@swissonline.ch", "<Passwort>", "Test", "Test");
        }
    }
}