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!!!");
}