Found
They claim to be E2EE + give 25GB free forever. Very generous free plan.
Only some of their crypto library appears to be open source at the moment. Nothing else.
The ML-KEM part of which is a re-implementation of a Typescript library, which is a re-implementation of a JavaScript library, which is a re-implementation of a Go library, which is a re-implementation of a C library.
Maybe one for the watchlist. But I wouldn’t even consider using it at the moment.