This depends what application you have set up. Assuming it is a forum, then No!
By their nature forum software leaves it to the user to make the decision on whether they want to receive email. They need to "subscribe" to get an email copy of each post made. The best you can do is send "private mails" on a one by one basis.
However, if you have set up a mail list, then, by its nature, anything you post will be sent to all subscribers.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.