It’s not completely FOSS, but I run Port87, which is quite a bit FOSS. It uses Haraka as its SMTP server, SvelteKit as its server framework, Nymph.js as its database layer, Svelte as its frontend framework, and Svelte Material UI as its UI framework.
The ones that I created and maintain are:
The base app layout is also available on GitHub.
Ultimately, you can’t. Even if everything you’re doing is encrypted, they have access to the RAM that’s holding your encryption keys.