Only part of it is FOSS (the UI basically), the sync engine is closed source and using Mailspring requires creating a Mailspring account FWIW.

I wonder what's in their privacy policy. I don't mind paying but wish everyone didn't use the same semi-copy pasted privacy policies for liability reasons.

Edit: typical 'your pii is an asset we transfer when we get acquired' clause

Information Shared with Third Parties. We may share aggregated information and non-identifying information with third parties for industry research and analysis, demographic profiling and other similar purposes.

Information Disclosed in Connection with Business Transactions. Information that we collect from our users, including PII, is considered to be a business asset. Thus, if we are acquired by a third party as a result of a transaction such as a merger, acquisition or asset sale or if our assets are acquired by a third party in the event we go out of business or enter bankruptcy, some or all of our assets, including your PII, may be disclosed or transferred to a third party acquirer in connection with the transaction

Doesn't it use MailCore and SquirrelMail?