There are currently three publicly available mailing lists for Geronimo:
The user mailing list is for users of Geronimo to share their experiences and ask questions. Many of the developers monitor that list as well, to make sure questions don't go unanswered. Currently this list has relatively low traffic, since Geronimo has not yet published a 1.0 release.
The developer mailing list is used by the developers to discuss plans, make decisions, vote on technical issues, etc. It also gets change notices whenever issues are added to or updated in JIRA. This list has a higher volume, though some conversations take place on IRC as well.
This list receives notifications (with diffs) every time a change is committed to the Geronimo source tree. It also receives change notices for changes to the Geronimo Wiki.
The Geronimo PMC has a mailing list to resolve any administrative issues, and the group working on TCK has a mailing list to discuss compliance issues. The first is limited to PMC members (most committers on the project), while the second is limited to committers who have signed an NDA to work on the TCK compliance effort.