There are many active discussion venues for Jabber/XMPP developers, system administrators, and end users. These lists are all easily searchable at jabber.markmail.org.
For End Users
JUser@jabber.org email list — for discussion about use of IM clients (subscribe | archives)
For System Admins
Operators@xmpp.org email list — for discussion about day-to-day operation of the Jabber/XMPP communications network (subscribe | archives)
For Developers
JDev@jabber.org email list — for discussion about software development using Jabber/XMPP technologies (subscribe | archives)
Standards@xmpp.org email list — for general discussion about XMPP protocols (subscribe | archives)
xmpp@ietf.org email list — official list for XMPP Working Group discussions at the IETF (subscribe | archives)
Special-Purpose Venues
The following special-purpose mailing lists provide low-volume, high-quality discussion among developers interested in specific aspects of XMPP technologies.
IOT@xmpp.org email list — for discussion about the use of XMPP in the Internet of Things (subscribe | archives)
Security@xmpp.org email list — for discussion about security issues related to XMPP, including encryption, authentication, and spam (subscribe | archives)
All of these venues are completely free and open to any interested individual.
