Posting by emails and getting updates by emails are the two most important things IMO for a small community site, or community which does not have tech savy users or “low internet users group”

If users can post in forums via emails and get updates to the forum post in their inbox the site usage will go up a lot.

This feature is not available in many CMS and even vbulletin does not have a module to date. So Drupal has a edge here.
Modules required:

1. Mailhandler
2. Listhandler

Configuration required on webhost or mail host for.

1. Setting up a mailing list.
2. Setting up an email.

It is a bit time consuming to set it up, but its worth it. Check out the details at