SMTP 535 5.7.3. indicates that l,.Authentication. SMTP client failed. You must identify the target server, the account used and the expected method — for example.SMTP AUTH. or OAuth — before changing the security identifiers or settings.
What does this error mean?
SMTP server refuses identifiers or method of authentication.
Probable causes
A correct password does not guarantee that the method is allowed. Failures can come from a deactivated protocol, a security policy, a bad hold, a blocked account, or a client who does not implement the method expected. Avoid repeated attempts with the same parameters: they complicate logs and can trigger other protections.
Verification and resolution
Follow the server name, port, TLS and the account actually presented, then check if this account can use the selected authentication method d. In Microsoft 365, check the AUTH SMTP settings for the holder and box as well as log-in logs.For OAuth, check the token, its audience, permissions and I, identity used by the application.
How to validate the correction — SMTP 535 5.7.3 — AuthenticationFood.
First, select the account and method on a client or a controlled test, then reapply the configuration to the application. Store the full SMTP message with time stamping and session ID when the server provides one. A correction is confirmed when the authentication succeeds and a test message is accepted without bypassing the control of the holder.
FAQ — SMTP 53557.3 — Authentication.
535 5.7.3 does it mean the password is false?.
Not necessarily. The method of authentication can be banned or misconfigured.
What to check on Microsoft 365?.
The account, the applicable SMTP AUTH parameters and the actual authentication method.
Do we have to disable the protections to test?
No. It is better to identify precisely the policy or protocol that blocks and use a supported method.