Microsoft 365 Alert – Service Degradation – Exchange Online – Users are unable to send outbound email messages in Exchange Online – RESOLVED

11/04/2023 09:10:00 AM (GMT)

NHSmail Reference : INC35246397

Microsoft Reference : EX539594

Issue Status : RESOLVED

Issue Description : Users are unable to send outbound email messages in Exchange Online.

More Information :Impact was specific to sending email messages to addresses outside of their organization. Email messages sent to other users within the organization weren”t impacted.

Users saw the following within the NDR message: “Remote server returned ”550 5.7.705 Access denied, tenant has exceeded threshold”

Affected users would have encountered this issue in all Exchange Online connection methods.

Final Update : 11/04/2023 09:12:00 AM – Microsoft have confirmed through user feedback and internal telemetry monitoring of NDR occurrences that disabling the problematic anti-spam rule has resolved the issue.

Scope of impact : A recently updated anti-spam rules wasn’t processing or evaluating email data as expected, resulting in impact.

Root cause : Daylight Saving Time triggered a latent code issue that’s causing non-compliant data to be sent when webinar times are updated, resulting in 400 errors.

Next steps : Microsoft are reviewing their anti-spam rule update processes and testing procedures to help prevent problems like this in the future prior to implementation. They’re additionally evaluating telemetry indicators, such as NDR increases, to better detect problems like this.

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