A mailbox rule is an automated instruction set within email services that processes incoming or outgoing emails based on predefined criteria. Users can configure mailbox rules to automatically sort, move, delete, forward, or flag emails according to specific conditions such as sender, subject line, keywords, or attachments. These rules help users manage their inbox more efficiently and reduce manual sorting. In cybersecurity, mailbox rules can be exploited by attackers to hide malicious activity, redirect sensitive information, or maintain persistence after compromising an account. Regularly reviewing and managing mailbox rules is essential for maintaining email security and preventing unauthorized actions.