This mailing list to support DITA was set up as a Yahoo Group by IBM in May 2004 and called the IBM DITA Forum.
Main page: https://groups.yahoo.com/group/dita-users/
Post a message: dita-users@yahoogroups.com
Subscribe: dita-users-subscribe@yahoogroups.com
It now has over 4000 members and is the main mailing list for the DITA community.
Links to several other DITA-related mailing lists can be found here.