By placing an opt-in link on your homepage, visitors to your site can subscribe to your newsletter to stay updated with useful content.  This is a great way to keep your current audience active by reminding them you're there and keeping them interested.  Don't overdo it, however, and don't give in to the temptation to spam.  You'll soon get an adverse reaction and your site will be given a wide berth.

Dotmailer  advanced design tools, an easy-to-use interface and campaign management features, you can send HTML e-mail campaigns, newsletters, weekly specials, support tips or marketing messages in an effective and efficient manner, providing each of your customers with personalised attention and the high levels of support.

FreeMailer2000 - Free Multi Level Mailing List Builder Once a new member has joined, they agree to receive messages from their up line in exchange for being able to send mailings to their down line. To ensure protection against spam, a small message is added to the bottom of every message members send stating that they are receiving the mailing because they are a member of the FreeMailer2000 system. Good in principle.

Microsoft Advice Centre  Articles on this subject, but don't stop there, MS has quite a useful suite of advice pages and resources for the web builder.