I can send and receive iMessages from all of my friends except for one person. She just got her iPhone yesterday and has all the right settings on. I am also the only person that she can not send iMessages to either. What can be the cause of this?
Registration error on her phone number. Try sending the iMessage to her email account that is registered. If that works, have her turn off iMessage. Reboot the phone. Turn iMessage back on. That "seems" to work if the email iMessage goes thru.