Office365 intermittent deferred messages with: "450 4.7.9 Application queue quota exceeded"
Date: 02 Jun, 2026 18:25 GMT / 14:25 EST
Starting around 14:30 GMT / 10:30 ET today, Some customers have noticed that some Office365 domains are not accepting messages periodically with the following SMTP Response:
450 4.7.9 Application queue quota exceeded
Very likely related to the following Office365 Service health announcement from around the same time:
https://admin.cloud.microsoft/?#/servicehealth/:/alerts/EX1331830
It looks like it is not system-wide, but affecting a small number of domains, and it looks like it is intermittent (i.e., the next retry of the message, it gets accepted by Office365.
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Update: The last incidents of this were around 2 Jun, 22:15 GMT
This is the update from the Service health report:
Jun 3, 2026, 2:07 PM EDT
After a period of extended monitoring, we've confirmed that customer impact was fully resolved.
- JunAfter a period of extended monitoring, we've confirmed that customer impact was fully resolved.
- 026, 2:07 PM EDT