mailbox.org, Posteo and @Tutanota@mastodon.social are secure, data-friendly and established.
They each have their pros and cons. For example posteo has a lot of payment services, I think tuta has a own client, and mailbox.org is really clean AFAIK.
Garnicht mal so pädagogisch wertvoll, Kindern zu zeigen, dass "wir Tiere beherrschen können". Ich weiß nicht warum das in den Kontext von naturnähe gebracht wird.
Let's dream big: how would you organize and pay a huge amount of open source software? Tech project giants?