Email client for android/ios (@decipher.email)

Android/ios application similar to ProtonMail client, send and receive email encrypted to/from server. From server mail transport is via secure smtp. Messages received by mailserver (qmail?) is encrypted with users key and later downloaded by email secure pop protocol - then decrypted on client app.
Messages sent stored in users outbox, encrypted with key, transported to server outbox by rsync or similar - decrypted on server with users key and transported by regular smtp.
Mobile mail client should be able to filter incoming messages by subject or sender, store unlimited messages on device, attachments to saved to folder, structure as regular folders if possible with filter.txt at root folder so it's possible to backup/restore ro synchronize with other device (if practical?).
Check new messages by sockets, not complete pop mail authentication. New email to server should trigger database to send socket message to start pop authentication/download new messages.