What is currently the best practice for storing secrets for a .NET app? My app implements a mail client (among other things that also require storing secrets), and storing them in plaintext feels bad.

Ideally I'd like to not go overboard on what I'd have to implement to do it right, I do not want to add a package dependency, and I'd ideally like to do something that isn't explicitly Windows-specific, since I'd like to someday claim to be cross-platform.