This type of feature is one that you see everyday users ask for all the time, but folks developing web forums tend to dislike because it can significantly complicate the administrator’s life with mailserver issues.
Sending out e-mail digests or alerts about new posts one-way is usually quite standard.
However, if someone really wants something open source that works two-way (instead of being locked into Google/YahooGroups or lacking a true web forum look like Mailman or Sympa), try GroupServer: http://groupserver.org (Note that it is Zope-based)
We use it here: http://forums.e-democracy.org
Steven Clift
E-Democracy.Org
http://e-democracy.org/blog – Happy WordPress users.
