In Germany, businesses are required to immutably archive all e-mails (except if they contain personal data unrelated to business operations, in which case they must be deleted).
The guidelines for this are specified in the GoBD (Grundsätze zur ordnungsmäßigen Führung und Aufbewahrung von Büchern, Aufzeichnungen und Unterlagen in elektronischer Form).
German mail providers offering services for businesses therefore usually offer conformant archiving services, e.g. IONOS: https://www.ionos.de/office-loesungen/email-archivierung
We are unhappy with our current E-Mail service and would love to switch to ProtonMail, but are unable to do so as long as this is not possible.