Can you turn off suggested emails that are external in OWA?

[kennysarmy]

I've been asked (after the wrong parent was emailed by a member of staff) if it's possible to turn off "external" suggestions when you start typing an email in Outlook Web App.

 

As per the attached, I've been asked if it's possible that only "internal" email addresses are shown in the suggestions but not "external" - I'm not sure this is possible, but thought I'd ask in case anyone knows....