You may set up a routing rule which sends out the bounce back message as follows
Set up a service to manage this type of emails
Create a routing rule which logs a request against the service created in step 1 above
Create a business process which assigns the call to a team, sets the status to close and sends out an email with your required message e.g. "This mailbox is not monitored. Please raise all requests via the Portal"
Ensure the rule created in step 2 above is the last rule processed by your email routing
Hope this helps