You need to tell the SMTP client that you will not be using your windows credentials to access the SMTP, so add
smtpClient.UseDefaultCredentials = false;
above this line of code
smtpClient.Credentials = new NetworkCredential("[email protected]", "password");
Also, gmail doesn’t allow impersonation, so
mailMessage.From = new MailAddress("[email protected]");
will have no effect – the emails will still appear to be sent from the email account you are accessing.
Also, make sure that the setting “allow less-secure applications” is set on your gmail account, and that 2-factor authentication is not enabled.