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
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