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.
GroupServer is a Web-based mailing list manager designed for large sites. It provides email interaction like a traditional mailing list manager but also supports reading, searching, and posting of messages and files via the Web. Users have forum-style profiles, and can manage their email addresses and other settings using the same Web interface. It has supports features such as Atom feeds, a basic CMS, statistics, multiple verified addresses per user, and bounce detection, and is able to be heavily customized.