Hi Gregor,
click on their forum link, it explains the background and how to verify, if you’re already registered.
Everyone got this notification, independent if he has registered or not.
Btw. the forum message also has a link to their privacy policy, one of the things which was missing when we (EGroupware GmbH) first looked at their push service. They claim to be fully GDPR (DSGVO) compliant and I have no reason to daut them.
Ralf