Recently we received complaints from our programmers saying their codes to send emails via smtp.office365.com were failing due to login failure.
We checked and found that the issue is due to the introduction of Security Defaults in Azure security to block legacy authentication methods. As a quick solution, we resorted to turning off the securiy defaults temporary while learning more about it. We found that for best practice we’re required to relay our mails through our own SMTP relay allocated together with our Office 365 setup.