Only allow sending mail from certain domains

Mail is being sent by MailChimp who does not know if you are working on a development/staging server.

This could result in accidental mail being sent to subscribers.

Proposed solution:

Add a config setting textfield to allow for comma seperated domains which are allowed to trigger the sending of mail.
These domain will be checked against the $_SERVER['SERVER_NAME'] php variable.

This required field will also be validated when submitting the settings form and displaying the current status as a $message.