Focusing on reactivating their disengaged users is likely not the most effective use of their time at this point. With a relatively young product, they're probably right to be focusing their time and energy on filling in product gaps and letting their audience know about it.

There is a time and place for complex, context aware email marketing, but they're probably not there yet.

That being said, it wouldn't hurt to start off with a simple sentence that reminds/reaffirms the value of their product to everyone.