Yep, it is built in, but many (including myself) disable it. It's pretty common that I accidentally open a message, but I don't have a response / a chance to respond yet, and I don't want the person to think I'm ignoring them.
In fact, SMS already has this system in place. Naturally there are some networks/phones that don't support it, and its meaning can vary from "network received" to "phone received".