At the moment, JIRA will throws in the logs if the reporterusername is not set and the createuser is false:

 Sender is anonymous, no default reporter specified and creating users is set to false 

There is no indicator that JIRA has receive email from un-register email address. This might cause very important message from user(who is sending email from a different address) did not pass through and the Administrator are unaware of this unless manually checking the logs. This is very reactive.

It's great to have similar feature to Bulk Email to address this. For example, Forward the email to a certain email address if the sender is un-register email address.