You are probably trying to use a different email address than the one her device is registered for. Ask her to go into her Messages settings and tell you what email address she is set up to receive with.
If she is using an iPhone, it is possible she has no email addresses set up. If so, have her turn on the ability to receive messages on one of her email accounts. Or you can use her phone number. Phone numbers work to connect to iPhones. It will even cross over and use SMS on the iPhone when the internet is not available.
In any case, the problem is probably on her end. You are getting the warning because the address you are sending to is not recognized by the iMessages server. That would not affect her ability to start a connection with you.