What I do use though, is mass PMs.
With those you can reach your members while they are browsing your forum. And those who are not browsing the forum, will get a notification e-mail.
In my own forums I have only clocked 2 or 3 people who turned off PM notifications via e-mail.
But just to cover both bases, I would suggest a mass PM and mass E-Mail feature. And that "can't reply to this message" option is really good for those mass PMs.
I did run a search and haven't found anything with "mass" in it - so I'm hoping this one isn't a duplicate. If it is, I apologize.