There are a couple of things that you can do to make sure that everyone gets the newsletters.
The first option is questionable, and may bother people who decided to opt out of the newsletters/emails from your site. You can click on manage site, settings, manage members, and then click on the "export members" button to get a .csv file that will contain a full list of all the member data, including a column that contains all emails. You can copy all of those emails and then send the newletter out from an email client ex(google, yahoo, etc). Using this option, you'll want to send it out with the bcc option.
Then, there is another thing that really helps to make sure "most people" if not all get the newsletter. When members sign up, the box that needs to be checked to opt in for emails is unchecked. Most people don't take the time to fool with that box one way or another, so if it was checked, more people would get your newsletter.
There is a plugin to make the checkbox checked by default instead of unchecked. You can find that here.
