@cathygellis I concur with @AndrewHenry that Thunderbird is a suitable alternative for most. However, that's only the easy half of the problem. The email and (if shared among devices) contacts and calendars still need to be hosted somewhere online.

I'm still looking for an end-to-end encrypted email, contacts, and calendar host with an open API for bridging desktop email clients, so I can have all my email and calendars in one place without any vendor lock-in.

So far, @protonprivacy offers Proton Mail Bridge exclusively for paid Proton Mail subscribers, but that's inadequate, as it's proprietary and incompatible with competing services such as @Tutanota and @mailbox_org. As far as I can tell, neither of those competitors offer any bridge options at all, and an email and (especially) calendar host is useless to me if I can't access all accounts—personal and work—across services in a single interface.

Meanwhile, none of those services offer #E2EE cross-compatibility, which makes their E2EE offering next to useless, as it's only effective within their respective walled gardens. (I expect zero prospective clients would migrate service providers in order to do business with me, and I'll no sooner maintain multiple concurrent email and calendar hosting subscriptions than I would multiple concurrent streaming video entertainment services.)

A non-proprietary shared API and a single open-source bridging program for all three (and any other competitors) would suffice for me to migrate all my personal and business email and calendars to one of those services. E2EE competitive compatibility, in addition to that, would suffice for me to begin guiding all my clients to migrate off Google and Microsoft. None of the three seem to realize yet that their main gains would be at the expense of Google and Microsoft, not each other, at least insofar as their lack of competitive compatibility indicates.

But that's all tangential to your question. If you want a not-terrible email and calendar client on Windows, the only option I'd consider is Thunderbird. Although eM Client seems to be well developed, I'd never consider proprietary desktop software for email or calendars for the same reason I'd never consider a proprietary desktop web browser: replacing Windows with Linux (or BSD) is relatively quick and easy, if that becomes necessary, but migrating accounts, mailboxes, rules, contacts, and calendars; translating configuration; and developing and learning a new workflow for the new email and calendar client would a major business disruption for anyone (like me, and I'm guessing for a lawyer such as yourself) to whose workflow email and calendars are central.

If eM Client Inc. decides to impose AI features in a manner which conflicts with your or my confidentiality obligations to our respective clients, we may only find out retroactively, and may while dealing with the fallout of that breach, we'd still need to urgently migrate everything to a new client. If the Mozilla Foundation decides to push similar misfeatures into Thunderbird, however, we'd be able to determine that beforehand, and it wouldn't take much for drop-in Thunderbird replacements (analogous to LibreWolf or Waterfox) to be released and adopted, because Thunderbird is open source. Transferring all data and settings from Thunderbird to such a replacement would be as simple as renaming one folder.

@landesregierung Klasse. OpenTalk ist genial. Wer das als Privatanwender nutzen möchte: Ist Teil der bezahlten Version vom Mailanbieter #Mailbox.org. Wer sowas nur für 5 Leute und 30 min braucht, kann es auch kostenlos direkt bei OpenTalk bekommen. Wer als Privatanwender mehr als 5 Leute hat oder länger als 30 min reden will: #Adminforge hostet das kostenlos. Nur der Meetingstarter braucht ein Konto, der Rest kann via Gastlink teilnehmen.

@teetassemitsprung
Hinter #eclipso scheint nur eine Person zu stehen die auch noch andere Sachen macht. Wäre mir zu heikel. Lieber eine Firma die Mail anbietet und sonst nichts. In D würde ich #posteo.de und #mailbox.org empfehlen.