There's a 30-day trial to use a version where the hosting is paid for by Nylas. After that point you need to upgrade to Pro in order for us to keep running the server infrastructure for you, and also to get features that depend on Nylas Cloud APIs.

The sync engine is open source, so you can also spin up a VPS on AWS, DigitalOcean, etc. and manage it yourself there.

You can read a bit more on the blog post: https://nylas.com/blog/nylas-pro