The mailing list for SWI-Prolog specific issues is a Google group. This list is used for announcements as well as questions and discussion amoung users. On average, traffic is 5-10 messages per day. Useful info:
Before being moved to a Google group (May 1, 2014), the mailing list was kindly hosted by the University of Bonn. The archives of this list are still available here.
Bugs are best reported to and tracked through the GitHub issue tracker