We have an older Windows Server 2012 machine running the IIS 6 SMTP service as an internal relay for devices that send mail through our Exchange Online tenant. Most messages use either a noreply address or a support address. Since August 27–28, the noreply address has stopped sending successfully, while the support address continues to work. Both appear to use the same IIS SMTP outgoing configuration. The relay log shows a successful connection, STARTTLS negotiation, and an AUTH attempt, followed by Exchange Online returning 535 5.7.139: authentication was unsuccessful because the request did not meet the requirements for authentication. What should I compare or check to determine why only this sender is failing?
3 Answers
The 535 5.7.139 response points toward an authentication or policy problem. Check the account used for SMTP AUTH, including whether its password, sign-in restrictions, SMTP AUTH setting, or conditional-access policy changed. Since the other sender still works, compare the working and failing transactions rather than assuming the whole relay is broken. Also keep in mind that basic authentication for SMTP is being phased out, so this setup needs a longer-term replacement or migration plan.
The two-hour difference in the IIS timestamps is probably unrelated. W3C-style IIS logs commonly record events in UTC, so the log time may differ from local time by two hours. The SMTP server's 220 response shows the session is reaching Exchange successfully; the actual failure occurs later during authentication.
A failure affecting only one address can also come from the sender details rather than the relay credentials. Compare the envelope sender, From header, authenticated account, connector restrictions, TLS settings, and source IP for both messages. A mismatch between the authenticated mailbox and the noreply envelope sender may be rejected by the tenant. Testing the same credentials and source IP with a minimal SMTP client can help separate an IIS configuration issue from an Exchange Online policy issue.
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