Another nice thing about most of these automated systems is that you can set them to do "double opt-in".
This means when someone signs up for your newsletter via your website, they autmoatically get an email asking them to click a link to confirm their subscription. Only then are they added to your mailing list.
This helps lessen the risk of you spamming people because it gets round the issue of people going on to your site and just adding loads of email addresses that don't belong to other people.