Did you put 170 addresses in the To: field? You could try putting them all in the Bcc: field and put the list admin address in the To: field. This might trip some spam filters, too, because the user's email address won't be visible in the headers.

If that fails, it ought to be pretty easy to write a script to automate 170 emails one by one. Well, if you hadn't eschewed a real OS. I suppose perl is available for Win32.