iMessage will not send to one person...

Ok so I have the iPod 4th generation. It is up to date but heres the problem. I am unable to send an iMessage to a person. I can type it, however, once I press send, I never get the little "delivered" note.Even restarting or resetting internet connection won't help. This is only happening to one person that I regularly message. Everyone else I can message to. The person who I am trying to message has an iPhone 4 and I'm not too sure what happened because we were just talking to each other yesterday. What is wrong? Is it long distance? Is the other person's iPhone not on or doesn't have iMessage not on? My internet connections works excellently. My iMessage is on and the Wi-Fi on my iPod is fine. Everything is in good standings, but my messages to that one person will not deliver (and give me that deliver note) 😟