They're only encrypted if you're sending within each of their individual ecosystems...look at it that way as a precaution.

Protonmail to Protonmail 🔒

Tutanota to Tutanota 🔒

Skiff to Skiff 🔒

Fastmail to Hotmail 🔓 ‼️

Skiff to Gmail 🔓‼️

Protonmail to Yahoo Mail 🔓‼️