Office365 intermittent deferred messages with: "450 4.7.9 Application queue quota exceeded" : TitanHQ Helpdesk

Office365 intermittent deferred messages with: "450 4.7.9 Application queue quota exceeded" Print

Modified on: Tue, 2 Jun, 2026 at 7:30 PM

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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Office365 intermittent deferred messages with: "450 4.7.9 Application queue quota exceeded" Print

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