No FROM Sender--How is that possible?

I received an email the other day. In my inbox list, it was listed as (unknown). The message basically was sent from a Verizon phone. The body just said: "hi tia hru?'

HRU must obviously be text shorthand for HOW ARE YOU, right?

Anyway, that's not the problem/question.

I hit REPLY to reply and there is NO FROM address so I have no idea who sent it.
How is it possible to send something to someone without a SENDER address?