The mailing lists have been consolidated into one Google Group for users and developers asking questions about XStream's usage, making enhancements requests or proposing improvements for its implementation. Another read-only Google Group is for all kind of notifications like announcements, commit or build server status messages.
Google Groups provide standard mailing list functionality as well as a user interface in form of a web-based forum.
The former mailing lists hosted at Codehaus are gone, but still available in public archives at MarkMail.