Como observado por Appleoddity em este comentário :
Is josh external? Do any other emails from the same domain as josh get delivered correctly? If you moved all the mail to the inbox and seconds later it moved back to spam, I think you eliminate many many things. I’m pretty sure both exchange and outlook classify the email when it arrives. It will not move it afterwards. You can also eliminate the entire mail path of arrival. It sounds to me this user has something on some connected device moving this email to spam. Is there a phone or anything connected to the account? You need to identify all connected devices and eliminate them one by one.
O OP xR34P3Rx respondeu com o seguinte comentário :
Turns out, even though Bob and Josh are internal within the same domain, Bob has a phone connected to his email, although he rarely uses it. It was his phone all along.