Add "and receive email notifications" to Alert Preferences.
Habits don’t form overnight. It takes several days, often weeks for a product or service to earn unprompted user engagement, triggered by people’s day-to-day emotions. Consider your use of Facebook, Twitter, Pinterest, or other popular, habit-forming products. Engagement starts with external triggers that inform the user what to do, driving the desired behavior.
Immediately after signing up on Twitter, the service recommends users to follow. Soon after, email notifications are sent to the user, highlighting tweets from those followed. Companies promote their @Username through TV commercials, billboards, online advertising, and even their own business card. Friends and colleagues talk about breaking news they discovered on Twitter. All of these external triggers - directly or indirectly delivered by Twitter - re-engage users.
This sounds obvious and some argue companies are too aggressive with external triggers (and some are), but it’s important to realize your Forum is of very little interest (at least initially) to others. Users are inundated with the distractions of everyday life - they need and want to be reminded you exist.