imessage is sending back an automatic message "Sorry, this service is not available"

When I text my friend's iphone, it sends as a text instead of an imessage and sends an automatic message back to me "Sorry, this service is not available." When he texts me, it comes through as an imessage, and I am able to message him from that message thread. My phone has not had problems with imessage with other phones,and he has not had problems receiving imessages from other people.