Microsoft 365 Alert – Service Degradation – Exchange Online – Some users may be unable to send Exchange Online email messages – RESOLVED

18/07/2023 08:32:00 AM (GMT)

NHSmail Reference : INC37075572

Microsoft Reference : EX649175

Issue Status : RESOLVED

Issue Description : Users may be unable to send Exchange Online email messages.

More info : Affected users may receive the following error “503 5.5.1 bad sequence of commands”

Final Update : 19/07/2023 09:58:00 AM – Microsoft determined that a recent change caused a large influx of free/busy Control Flow Messages (CFM) to be sent to the Exchange online infrastructure, resulting in stale free-busy information. An interceptor rule was applied to discard these CFM messages to mitigate impact for the free/busy issue, which inadvertently caused impact to mail flow.

They have removed the interceptor rule and verified that the impact to the mail flow has now been mitigated. Microsoft will continue to investigate the free/busy issue, which is being communicated under the SI EX648815.

Due to the nature of the issue, the users who had received the NDRs during this event will need to resend those emails.

Scope of impact : Impact is specific to some users who are served through the affected infrastructure.

Root cause : Microsoft have determined that a recent change caused a large influx of free/busy Control Flow Messages (CFM) to be sent to the Exchange online infrastructure, resulting in stale free-busy information. An interceptor rule was applied to discard these CFM messages to mitigate impact for the free/busy issue, which inadvertently caused impact to mail flow.

Next steps : Microsoft are reviewing their interceptor rule implementation procedures to understand why mail flow was inadvertently affected, in order to help prevent similar impact from happening again in the future