Exchange Online blocked emails (June 18, 2023)

Stop - Pixabay[German]Today, July 18, 2023, Microsoft's Exchange Online cloud service appears to have been experiencing a [partial] glitch since morning (8 a.m. UTM). German blog readers reported that emails are being blocked. There are various references to issues on 3rd party status pages, and Microsoft's social media team also writes on Twitter that they are investigating reports of disruptions. Here's a brief overview.


The first information reached me from German blog reader Tom801, who left a comment in the discussion area, which I'll pull out here:

Exchange Online is rattling again EX649175 "503 5.5.1 Bad sequence of commands" especially it seems to affect DACH and UK.

On Twitter, @MSFT365Status also reports that they are investigating reports of an issue. I'll pull out the tweets in question as text:

We're investigating an issue where some users may be unable to send Exchange Online email messages due to a recent change to the free/busy infrastructure. We're identifying a plan to resolve the impact. Further details can be found under EX649175 in the admin center.

Users on Twitter are currently still denying information that the problem has been solved. The cause is currently unknown. I myself cannot access the status message EX649175 in the administrator area because I do not have a Microsoft 365 account. The Office365 status page does not report a fault. On this page, however, it says for the error EX649175:

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

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

NHSmail Reference : INC37075572

Microsoft Reference : EX649175

Issue Status : ONGOING

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"

Current Update : 18/07/2023 10:22:00 AM – Microsoft disabled an interceptor rule that we believe was causing impact to the mail flow. They are monitoring the service to ensure the mail flow is restored.

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

Next update by : Tuesday, July 18, 2023, at 10:00 AM UTC

However, the problem seems to have affected users all over the world – after a quick search, I saw that the colleagues from Bleeping Computer are also addressing the issue here. On sites such as, I saw an increase in the number of disruption reports from around 8:00 a.m. German time, although this is minimal with a peak of 25 reports per time unit.

In closed German Facebook groups I received mixed feedback. While one administrator wrote "didn't recognized anything", others reported issues. Here some feedback, which I've anonymized:


User #1: I was affected. We use ZScaler and ZCC. Since Monday, OneDrive no longer connects through ZScaler. So SSO is not working. Anyone else using this and noticing the problem?

User #2:Yep, we are affected. But only to certain mail addresses: Remote server returned '503 5.5.1 Bad sequence of commands ###param###

User #3:Yep, we are affected ourselves and also div. customers located at MS365.

The colleagues from Bleeping Computer reported here later in an addendum, that Microsoft has fixed the problem and deleted the entry for EX649175 from the Admin Center. Kind of a strange move, but I got also similar reports from German administrators wondering why Microsoft has deleted the entry.

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