OnINBOX checks all incoming mail to see if it has come from a sender you trust.
Senders can be manually trusted by you (or your organisation admin via OnINBOX Manager).
However, to get you started OnINBOX will scan through your historic contacts when your mailbox is first connected. Any contacts that you've interacted with will automatically be added to your trust network depending on an algorithm involved.
OnINBOX will continue to automatically trust contacts assuming their emails authenticate as being from the person they say they are.
You can always edit your trust network via the Trust Network tab in the app (accessible by clicking "Learn more" in any email showing an OnINBOX banner).