[Moderators/Assisants Only] Board > Manage > Manage E-Mail List
Every board has a built-in e-mail list. Pretty cool, eh?
People join it by clicking 'Subscribe to [boardname] E-Mail List]' at the bottom of the iBox/header.
Once you are on the E-mail List Page (within Manage), you can view subscribers, remove subscribers, and send out an e-mail to all the subscribers. Just type and click Send.