The mail user utility allows you to send email to all site users, or to a specific group of site users.
Users have the ability to select whether they wish to recieve site mails by selecting the Email from Admin option under My Account → Privacy Tab.
As a site admin, you have the ability to override this setting and force emails to users who have selected not to receive them. You should use extreme care when using this feature, if a user has said they do not wish to receive emails from your site, they may be upset if you ignore this setting.
Shared Hosting Environment Warnings
glFusion does not provide any method to meter or throttle the amount of email sent using this feature. If you try to send to all your site users, it will send the emails as fast as it can. Some shared hosting plans limit the number of emails you can send and using this feature may get you in trouble with your hosting provider.