> maybe this is because the emails are sent from s2survey.net?
Yes - as this is the paid server, we have disabled the automatic footer in mailings there. If someone abused the mailing feature on that server, we have a valid bank account to send the police to ;)
This is the default footer on www.soscisurvey.de (HTML version):
<p>This e-mail was sent using SoSci Survey (<a href="
https://www.soscisurvey.de/en/contact">details</a>). For information about content and privacy please contact <a href="mailto:%email%">%name%</a>.</p>
<p>If you do not want to receive further e-mails via SoSci Survey, please click <a href="%lock%">here</a>.</p>
As you see, I mixed up %opt-out% and %lock%, whereas the latter may be more approprite for your needs.