UPDATE: This topic is closed. All documentation and info about Newsletter is here.
##Solution in Place: "Ninja Forms"
On December 22, 2015 I installed on the oscedays.org wordpress a plugin called “Ninja Forms” to collect email addresses for newsletter sign ups. The form collects the email addresses in the data base of the wordpress - so you can find all sign ups to this point there.
All sign ups before were collected with the plugin Contact Form 7 that had send the addresses to an email account: firstname.lastname@example.org - But there was trouble with the form so we had to change. All the addresses we managed to collect that way were copied by @unteem to a document that is available in the owncloud.