A server that manages mailing lists for groups of users. Two of the most popular mailing list server systems for the Internet are Listserv and Majordomo.
There is also list serve software that you can buy for your PC. It all helps you to boost your sales by sending personalized HTML email and tracking live clicks, right from your PC. Some come with both live tracking, a live subscription manager, and WYSIWYG (what you see is what you get) HTML editing. And it helps you comply with anti-spam law by automatically handling unsubscribes.
Some sites also have free trials. Interesting you asked, because just last night I signed up for a free online trial at
http://www.verticalresponse.com/I'm in the process of building a trial newsletter to send out for my July sale. There is no subscription fee; you only pay for the emails or postcards that you send. The cost seems reasonable to me, since I don't do very many of these and don't have a large list.
If you do have a large list, and send out customized emails frequently, then the software you can purchase may be the better way to go.
Kathy