SMTP Error: The remote certificate is invalid according to the validation procedure

Try this

protected void Button1_Click(object sender, EventArgs e)
{
    MailMessage message = new MailMessage();
    message.To.Add(new MailAddress("phadke.shailendra@gmail.com"));
    message.From = new MailAddress("ContactUs@ShailendraPhadke.com");
    message.Subject = "Test Subject";
    message.Body = "Test message";
 
    SmtpClient client = new SmtpClient();
    client.Port = 25;
    client.EnableSsl = true;
 
    client.UseDefaultCredentials = true;
    client.Host = "smtp1r.cp.365techsupport.com";
 
    client.Credentials = new NetworkCredential("ContactUs@ShailendraPhadke.com", "Shailen5604");
 
    //Add this line to bypass the certificate validation
    System.Net.ServicePointManager.ServerCertificateValidationCallback = delegate(object s,
            System.Security.Cryptography.X509Certificates.X509Certificate certificate,
            System.Security.Cryptography.X509Certificates.X509Chain chain,
            System.Net.Security.SslPolicyErrors sslPolicyErrors)
            {
                return true;
            };
    client.Send(message);
 
    Response.Write("Mail Sent!!!");
}