Originally Nylas Mail was OSS, but tied to the Nylas Cloud APIs (which did the actual mail processing.)
The cloud strategy was too expensive, and in version 2.0 the entire mail-sync system was moved into the client and open sourced. (https://github.com/nylas/nylas-mail/tree/master/packages/cli...). It's now much more like Thunderbird. There's a bit of cloud processing for snoozing / send later, etc., but it no longer syncs all your email through the cloud.