Major Description
When editing the email alert settings it is not possible to disable email alerts while the 'Sender email' is empty, this seems illogical as if you are disabling email alerts you wouldn't care if the sender email field has a valid email or not.
You can press the 'Save' button however an error will appear next to the 'Sender email' field and the settings will not be saved.
There was similar behaviour in 4.1 and earlier, however in that case it was not even possible to press the 'Save' button while the 'Sender email' field is empty, again undesirable behaviour but at least it was clear that your settings were not being saved.
If we are disabling email alerts then we should not need to verify the contents of any of the fields as they will have no effect.