Limit the 'Subscribers' and 'From' suggestions to internal email addresses only

It would be great to be able to limit the 'Subscribers' and 'From' lookups to only return internal email address. 
External email addresses would still be able to be used by entering them manually or submitting the ticket via email.

We have 150+ Technicicans and thousands of external email addresses in our user list (e.g. an ex-staff member logs a Payroll request from their personal email address). These addresses shouldn't / don't need to be displayed to all technicians