Onet Poczta is the e-mail service of the Polish portal Onet.pl, a general-interest web portal with sections for news, business, games and lifestyle. Users sign in with an Onet account to open their webmail, which is reachable from the portal. For a European organisation, the relevant question is which group entity acts as controller.
The service is operated by Ringier Axel Springer Polska sp. z o.o., a Polish company with a correspondence address in Warsaw. The privacy policy names a controller within the Ringier Axel Springer Polska group, a data protection officer, and the GDPR (Regulation (EU) 2016/679). As a Polish company, the operator falls under EU data protection law. Organisations that need to know where mailboxes are stored should ask the provider.