Tuta is always encrypted I don't know where you got the impression that it was optional or that they could somehow magically make it work over IMAP without a bridge like proton.
So it "breaks end to end encryption" even with smtps and imaps apparently. The few emails I received weren't from tutamail users so presumably came over SMTP.
It's confusing to know what they mean because they confuse terms. They say emails are "stored end to end encrypted".
They don't pass my smell test