Intermittent Brevo “authentication not found in headers” errors while other emails send successfully

[osdsupport]

Hi,

We are seeing intermittent Brevo authentication not found in headers errors with FluentSMTP, mainly on background/system-generated emails such as WordPress automatic update notifications and weekly FluentSMTP reports.

• Brevo is connected and regular emails send normally.
• Test emails work successfully.
• We replaced the credential with a new standard Brevo v3 API key, but the issue still occurs.
• A recent affected email does not appear in FluentSMTP Email Logs at all, even though logging was enabled with 60-day retention.
• We also checked Brevo Transactional Logs. The affected email is not there, while other emails to the same recipient before and after it are logged normally.
• WordPress debug logs showed no related FluentSMTP, Brevo, or wp_mail_failed errors.

This appears to affect certain background/cron-generated requests rather than the Brevo connection generally. It may be related to #283.

Could this be an issue with Brevo authentication during background/cron email requests?

Thank you.

[osdsupport]

Another occurrence just happened with FluentSMTP’s weekly Email Sending Health report.

Slack reported authentication not found in headers, and there is no corresponding failure in FluentSMTP’s Failed Email Logs. However, Brevo shows the same email as both Sent and Delivered at the scheduled time.

So in this occurrence, the email was successfully delivered despite the FluentSMTP failure alert.