Sending encrypted mail by default (nothing to turn on)

Hello. I have set up K-9 mail with my nephews and nieces, and I want encryption enabled by default when they send email to me.

I can set up encryption by default in Thunderbird (on my end). Is such a thing possible on K-9 so that no unencrypted mails get sent (and nothing needs to be turned on, ever), because I am still getting messages from them that are sometimes unencrypted. This defeats the purpose of using encryption.

I am using the latest available K-9 on f-droid.org along with the latest available OpenKeychain on f-droid.org. Is there a way to make it impossible for them to accidentally send me a plain text email from K-9? Thank you!

I have set up autocrypt on each device but k9 still requires the user to press the padlock icon to encrypt, so it turns green. I want this to be automatic.

Can anyone help?

If K-9 cannot automatically encrypt reliably, is there a mail program for android that does? As stated above, sending mail from Thunderbird is flawless with encryption enabled by default. With K-9 and OpenKeychain it’s problematic.

For anyone that’s following this, the solution is to try another email app.

With encryption enabled by default in FairEmail, everything works as it should. No user input is ever necessary to turn encryption on. I dislike the interface in FairEmail, plus the fact that it has ‘pro’ features that you can’t use unless you buy it, but you can’t have it all.