Sympa is a scalable and highly customizable mailing list software. It can cope with big lists (780,000 subscribers), and it comes with a complete Web interface for both the user and the administrator. It is internationalized. Sites can customize Web and mail templates, authentication backends, and authorization scenarios. Dynamic mailing lists can use multiple backends (LDAP, SQL, files, lists, or others). Sympa is the only S/MIME and DKIM enabled mailing list software, and it provides both authentication and encryption.