> You can do this on desktop if you're using Bridge app with, say, Thunderbird. Thunderbird then will do the indexing.

Bridge is for paid users only, and it's not open source thing. https://github.com/vladimiry/ElectronMail enables full-text search support for free accounts if the "local store" enabled, described in https://github.com/vladimiry/ElectronMail/wiki/FAQ.