Spam filters don't need to store all the contents. They can simply store hashes of contents.

So even if someone hacks into the spam filters, they won't be able to reconstruct content from your account. They _might_ have insight into things like word usage, but they won't have reconstructed sentences or the like, a priori.

You can build systems like this without having long-term storage of the content, which protects against data issues for one-time leaks.