I found the solution on the Service monitor page, under alert rules: -

Under Server/Service section;

Enable - “When a server is disconnected….”

Under the Send Email Settings;

Add your email address in the ‘Send Email To:’ textbox and set the frequency.

This this is a good idea, the problem I found is when the HA server stopped backing one another of because the file systems became corrupted. Eventfully the Directory index became full on the Mongo Database server, causing the “mongodb” to go down. We need an alerts when this begins to happen so we will not go down again.