So, do they employ spam filters and they can still read messages before they encrypt them if the come from a third source, right?
The utility is not in magically making every email sent to or from you unreadable to 3rd parties. The utility is purely on the side or privacy intrusion: be it via court order or hacker. Nobody can compel them to decrypt any messages that have been sent to or from you and stored on their servers.
Protonmail will NOT: Prevent interception of your messages by 3rd-party MITM attacks.
Protonmail WILL: Protect your privacy versus legal authorities. Safeguard your stored communications against hacker breach. Provide a high-quality, ad-free email experience and multiple email addresses.