However, I wonder if there is a free (as in beer) publicly usable service which allows the creation of email lists. Users should be able to register /unregister themselves. Both, sympa and Mailman are ok (there might be more), but I am asking for a hosted service.
Email lists such as mailman are services used to communicate / share information. You can send an email e.g. to
email@example.com and the service sends the email to all subscribed students at KIT. They might also offer an archive, but that is not so important. I am only asking for the very basic service. If there are many free services which offer this functionality, then I might ask for more.