I think they just sent you en e-mail through the AirBnB e-mail gateway that hides e-mail addresses?
It used to be that you just got a guest's e-mail (and they got yours) but AirBnB is now using a gateway that hides e-mail addresses. This was one of the sillier changes for the worse in the last couple years, but it results in you getting e-mail that looks like it's from, for example "Xin (Airbnb)" <[email protected]>
That's nothing to be alarmed about. You can just reply via e-mail, or go back to the AirBnB messaging system and follow up there.