You consider them not as replies, although they come to you on the left side of the pane, so they are received messages. Hence me referring to them as being replies, my mistake.
If they are not received through Apple's iMessage system, then they are not encrypted. That is mostly the case if they are sent by external programs such as Joebob Motors Inc, who sends them from their computer system to an SMS provider which delivers them to your provider/mobile operator.
Your mobile operator can look into them, if they have a valid reason. Operators are not monitoring your messages if they have no reason to.