I compared mail providers a while ago for the same reason, and finally landed on Mailbox (mailbox.org) as they’re rather full-featured (web-based office suite, video meetings) and use standard protocols (IMAP and SMTP for mail, WebDAV/CalDAV/CardDAV for storage, calendar and contacts - in addition to the web interfaces).
Unfortunately, the site doesn’t work on Sailfish’s stock browser, but gets stuck in an infinite loading screen after logging in, but that’s pretty much par for the course. Fennec works well.
Welcome in the Unix / Linux world. Here are tools not “integrated” but they will “communicate” among each other. You can find more here: toolbox philosophy
This means for SailfishOS: you can use the nice native email app for mails. And you can use the nice native calendar app. They communicate together. And e.g. when you have setup a nextcloud account in settings you can add your nextcoud calendars to the native calendar app. But there are other calendars outside talking CalDAV. So - if you may love big companies - you can add (via CalDAV) your MS Outlook calendar. Maybe this magic should work also with an Mailbox.org, Apple or a Google calendar.
Wow, thanks for the warm kind welcome. I have been using Linux since the 90’s.
IF you have been stuck underneath a stone, a lot has happened in the world. Full support on Linux for Proton for one example. And I would say that the Linux on computer runs fantastic with almost everything.
If you cannot keep yourself from making pointless and besserwisser replies, I suggest not replying at all.
Apparently there is a way to run the Proton Mail Bridge on Sailfish, but as I said, I decided not to bother with it and instead use a provider that speak common protocols.
Apple accounts should work without real problems - maybe with the use of the little tricks one needed in the early days of Windows 10 Mobile (means: getting the right links for CalDAV/CardDAV).
(If I remember right I already used that with my C1 before we switched to Nextcloud.)