Germany
Open-Xchange is a German company founded in 2005 in Olpe and Nuremberg as an open-source answer to Microsoft Exchange. Its OX App Suite is the mail and groupware system that sits behind a large share of Europe's telecom and internet-provider webmail, covering mail, calendar, contacts and document editing at the scale those providers need.
You are less likely to sign up to it directly and more likely to already be using it through your ISP or telco. That makes it more of a sovereign backbone for providers than a consumer product, but it is one of the most widely deployed European alternatives on this list, reaching hundreds of millions of mailboxes.
