A mail spool is the on-disk storage area where a mail server keeps queued messages and related state before delivery. It may contain message bodies, headers, locks, and temporary files used by the mail transfer agent while processing mail.
In cyber security, the mail spool matters because it is a high-value filesystem boundary. If permissions are too loose, or if a bug allows directory traversal or path confusion, an attacker may read, modify, or delete queued mail outside the intended directory. That can expose sensitive content, disrupt delivery, or create a path to privilege escalation when the server runs with elevated rights. Defenders protect the spool with strict ownership and mode settings, confinement mechanisms, and careful validation of every filesystem path used by mail-handling code. Regular patching is important because spool-handling flaws often turn a small input bug into a serious local attack.