@Tutanota @edri Every client I support who's open to ditching Google apps has one indispensable requirement of any replacement of Google apps: They must be able to view their Google, Microsoft, and Google-replacement accounts in the same OS-native mail, contacts, and calendar apps. This serves two purposes:
They need to see their personal and work communications and schedules in the same window. This is essential, for example, to avoid double-booking themselves.
They need to be able to gradually transition between their old personal accounts and their new, as they update contact information with one person, business, nonprofit, and government agency at a time.
For #E2EE services, this requires a local SMTP, IMAP, CalDAV, and CardDAV bridge (or, for mobile devices lacking support for this, a self-hosted bridge and a VPN to which).
What options provide this capability?