If you head to Customers > advanced search and then set the filters to give you all of your customers, then hit the search button. On the resulting page, you will have a "send letter" option under the actions menu. This will allow you to send a form letter to that whole list.
On the resulting page, you will have a "send letter" option under the actions menu. This will allow you to send a form letter that you have previously configured under [settings >configuration > form letters] to that whole list.
That being said, I would recommend using MailChimp or another free email marketing system if you need to send out emails to a large number of people.