K-9 is an Android open source mail client. I've edited the code, so onion addresses are currently supported in my source. You may check the details in my blog: Connecting to Hidden Mail Server using Android
Additionally you can check my Pull Request in K-9's source at: https://github.com/k9mail/k-9/pull/3889
Hope it helps.
Well, it totally depends on the mail server configuration, not a client: you can utilize even a builtin one through Orbot with total proxification enabled. If the server is set up properly - it can even have an onion address! So - on the client side you need just a working email client, that's it! Everything else depends on server