After reviewing nearly all newsletter components for Joomla, I ended up with Mailster. It allows you to maintain mailinglists from the Backend, while the creation of the newsletter itself can be done from any member of the mailinglist without being a registered user of the site. It is the most easy way to allow communication between closed groups. The developer responds to questions very quickly and very satisfactory. With the already announced features like HTML support and the ability to copy mailinglists it will be a really must have tool for nearly every site. Thanks a lot for this great extension !