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K-9 mail is great, but just one part of the replacement.

I've yet to find a very good contact list for Android.

... and a calendar that works remotely as good as the one of Thunderbird (Lightning).

For calendar on Android, have you tried Etar? https://f-droid.org/en/packages/ws.xsoh.etar/

Thanks for the hint! I wasn't aware of that package. I'll try it.