AuthenticationFailedException: 535 5.7.139 Authentication unsuccessful error

The AuthenticationFailedException: 535 5.7.139 Authentication unsuccessful error is usually the result of turning off legacy authentication and enabling Multi-Factor Authentication (MFA) on the email server.

Diagnosing the issue

To diagnose this issue, check the logs: if the error code 535 5.7.139 appears in the Job History, this issue is present. No additional logging or vetting is required; the .139 is the specific code for this issue.

Fixing the issue

This Microsoft article explains how to fix the error on the email server software.

In Azure, the Conditional Access Policy blocks legacy authentication. Exclude the email account specified as the user account on the Informer email setup. This should fix the issue. If not, reach out to Informer Support for additional help.