The backend speaks SMTP, IMAP, and HTTP. It is basically a glue (or proxy) between SMTP/IMAP (including those protocols over TLS) and HTTP. Nylas Pro talks HTTP to the glue (I assume that is using TLS, so HTTPS). Nylas Mail also talks HTTP to the backend, but the backend runs locally, so you're self hosting the backend. I suppose this has a performance loss. (With Nylas Pro, you can set a separate backend server if you desire but that requires work. That is a really cool feature for a business though.)

So I would assume you are hosed. The passwords need to be stored plaintext.