While mail.protonmail.ch is not a honeypot itself, it serves ProtonMail, a privacy-focused email provider in Switzerland. This attracts a user base that's highly sensitive to spam and more likely to report it. ProtonMail employs stringent spam filters and security measures, potentially flagging legitimate emails. Senders face a higher risk of blacklisting and reputation damage if they don't adhere to the highest email marketing standards. Tools like MXToolbox can check server health, and services like Mailhardener can verify compliance with SPF, DKIM, and DMARC. Senders need to focus on permission-based marketing, relevant content, and providing value to avoid issues with ProtonMail users.