iMessage won't work with one other person?

The other day my iMessage stopped working with one other person and only one person. My messages would say 'delivered' but the person never received any of them. The only way I can text them is by turning my iMessage off. I've never had this problem before and it's not their imessage that is messing up because I still get their messages, they just don't get mine. I've reset my network settings and reset my whole phone. Is there any way I can fix this?