SharePoint Online People Picker Returning External Email Addresses

Question

Previously in our tenant when using the People Picker within SharePoint Online lists, only internal users would be returned in the search results. Now people picker searches are also returning external email addresses. Even though the external emails are showing within the search results, they can't be selected.

I can't figure out where SharePoint is finding these external email addresses, I believe they have originally been entered via Outlook so they are potentially stored in Exchange, however I can't find any trace of where the external emails are being stored.

Can anybody please let me know where this information is being stored? Also is it possible to restrict SharePoint Online's People Picker search results to only return internal AD users?

Many thanks,

Alex

EDIT: Using F12 Dev Tools I have been able to ascertain that the internal users have a subclass of "OrganizationUser", whereas the external users have a subclass of "ImplicitContact". Now the question is: where is SharePoint storing/pulling Implicit Contacts from?...