Yeah, there are multiple threads on this, I guess I should keep a copy my response as I keep posting the workaround...
This is supposedly a known issue by Apple that they are working on. The interim fix is to force the email app to close. To do this follow these steps:
1. Open email app
2. Hold down home button for ~15 seconds until the email app closes (this is force closing the app)
3. Go back into the email app and you should be able to send
4. If steps 1 - 3 fail, power cycle the phone
I have done the above 1 - 3 and it works to resolve the issue.
Hope this helps.
ETA: BTW, if you were trying to send a message and it wasn't sending, force closing the email app will result in the message draft you were trying to send to be lost. This is the one really annoying part of the workaround but as far as I am aware their is no way to prevent the message loss. You will need to retype the message and send again.