For decentralisation, ease of hosting and censorship evasion, #DeltaChat probably beats both #Session and #SimpleX. No blockchain. It uses tried and tested #e2ee-only email servers called #Chatmail, so it can work with any standards compliant app, like nostr:npub16zxpwcght62n7q4j88uj6ynehdqfzk572rues3y2tshr0vehfwjq2v4ynp, #Thunderbird, etc.