iMessage won't send?
Email and iMessages are generally speaking two entirely different things, with a few exceptions on iOS.

It is possible that this is because the person you are trying to iMessage either doesn't have an iMessage capable device, or doesn't have it enabled. However, because it will allow you to send, it I think it is more of a configuration question, potentially on their end. 

First check to make sure the recipient has access to iMessage as well. Also, verify that you are sending it to their Apple ID email, if they are on an non-cellular device such as an iPad, or iPod Touch. You should also try iMessaging someone else, to test it.