Exchange Online is returning “451 4.7.500 Server busy” across Europe
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CedarFox42
We're seeing an increasing number of temporary mail-delivery failures when sending to Exchange Online in Europe. The error is usually "451 4.7.500 Server busy. Please try again later," and messages are backing up in our mail filters, sometimes eventually delivering after 15–60 minutes. The Microsoft 365 status page and Downdetector aren't showing an incident. Is anyone else experiencing this, and has anyone found a reliable workaround?
Answers
We’ve opened a Microsoft support case because the errors are creating a significant backlog. It’s probably worth filing a case from affected tenants as well, since multiple reports may help Microsoft correlate the regional issue.
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Some people also tried adding a connector for their filtering service’s public IP, with reports that it helped. However, connectors can create routing problems when multiple organizations share the same filtering infrastructure, so this should only be treated as a cautious troubleshooting step rather than a confirmed fix.
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One organization reported that bypassing its filtering service and sending directly to the Exchange Online MX endpoint worked as a temporary workaround. That may help isolate whether the issue is related to the gateway path, but it isn’t necessarily practical for every environment and should be tested carefully.
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Yes, this is affecting multiple European locations, including the UK, Denmark, Sweden, the Netherlands, France, Finland, Belgium, and Ireland. Several organizations are seeing mail queues build up, with some messages eventually getting through after roughly an hour. It doesn’t appear to be limited to one filtering provider—Mimecast, Mailgun, SpamTitan, LibraESVA, and others have all been mentioned.
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We’re seeing the same 451 response from both our own systems and a third-party smart host. It looks more like a temporary Exchange Online-side problem or throttling affecting particular sending IPs than a complete service outage. The public service-health information may simply be lagging behind the incident.
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Answers are reviewed before publication.
CobaltGarden34: That connector change may cause problems for organizations hosted on the same filter, especially when they need to send mail to one another, so we’re avoiding it until there’s clearer guidance.