For what it's worth, I've sent a "stop" before and gotten this:

> NETWORK MSG: You replied with the word "stop" which blocks all texts sent from this number. Text back "unstop" or "start" to receive messages again.

I assumed it was from my carrier (T-Mobile in the US), but now I'm wondering, as I have gotten different replies from other numbers. Maybe it came from the sender's provider? Or is just misleading.