Business email and groupware cover mail servers, shared calendars, contacts, resource booking and often file sharing and chat. They are the daily channel for contracts, customer correspondence and internal decisions, and many organisations also use the mail address as the login for other services. Microsoft 365 and Google Workspace are the usual reference points.
For this category the sovereignty question is concrete: mailboxes hold years of accumulated business records, and the provider normally operates the encryption keys. Check whether the vendor offers customer-managed keys or end-to-end encryption, and what exactly that covers, since search, spam filtering and webmail usually need access to message content. Migration is the second pressure point. Mailbox export, calendar and contact portability, and support for open protocols such as IMAP, CalDAV and CardDAV determine how hard it is to leave later.
When choosing a European option, also look at how well it works alongside the mail clients, mobile devices and directory your staff already use, at deliverability reputation, and at whether self-hosting is possible if requirements change. Ask for the hosting locations of backups and support access, not only of primary servers.