One user in my domain doesnt get verification email using Exhange online OWA

why is one outlook user not receiving email verifications. They are receiving internal as well as external emails but cannot get email verifications. The emails show in the applications as leaving their servers but a message trace in Exchange Admin Center shows the verification emails as never arriving, not in quarantine or spam or anywhere. How can they possibly get lost like this? We verified on backscatterer.org that neither our ip address nor her email is listed in on backscatterer.org