> I maintain that it would be nicer that the UI shows a red message explaining that the user (or admin)'s email is not defined (and tell him how to fill it) and grey out the ability to send the message till the user has defined his email address. Alternatively, when the user's email address is not defined, offer an input text to let the user enter it.

In the light of the last comment, i'm only talking about the admin_email not being defined.

I'm not sure it's such a good idea to block the user, waiting for an action that he cannot do himself. If he cannot control the fact that the admin has filled in the admin mail, it's bad to block him, especially if there can be a fallback.