Etar (an open source app for Android and iOS)

Kolab Now

Lightning Calendar

Mailbox.org

Mailfence

Posteo

Proton Calendar (powered by Proton who also have Proton Pass, Proton Authenticator, Protonmail, Protonmail Bridge, ProtonVPN, Proton Drive, a “privacy-first” AI called Lumo, and SimpleLogin.)

Tuta