Hi @Thanh Ngo ,
535: 5.7.3 Authentication unsuccessful
For this error, on the Exchange online side, in addition to enabling authenticated SMTP submission, please check the following points:
1.Disable Multi-Factor Authentication (MFA) on the licensed mailbox that's being used:
- In the Microsoft 365 admin center, in the left navigation menu, choose Users > Active users.
- On the Active users page, choose Multi-Factor Authentication.
-
- On the multi-factor authentication page, select the user and disable the Multi-Factor Authentication status.
2.Disable Azure security defaults
3.Check if you have set to exclude users in Conditional Access policies that block legacy authentication
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread