Hey! Mailspring maintainer here—the app doesn't support plaintext emails (at least not out of the box), but it's likely we'll add support in the future.

A lot of the features in the app, like read receipts and link tracking, are targeted at sales and business folks that send a lot of email and care about it being read. You can use the app without using them though! Unlike Nylas Mail, Mailspring implements all mail sync on your computer - your email credentials are not sent to the cloud and things like "unsnoozing" happen on your machine, not on a server. (Note that for many people, this is actually a /bad/ thing - if enough folks complain we might add the cloud sync stuff as an opt-in!)