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Microsoft throttling connections

Are other people that deploy Email Security seeing problems when sending email to Exchange Online? Every few months Microsoft starts throttling our ES IP addresses. Now it has happened twice in one week. Email Security queue shows "status 4xx, retry later" for dozens of messages. After a few hours when the retry timeout passes, users get non-delivery notifications.

I've battled with Microsoft support for months about this issue. They sometimes seem to be able to do something that releases the throttling on those IPs and the messages get delivered. But it always comes back sooner or later. Most of the first level support agents don't even understand what the issue is and start asking if we have connectors created on the tenant, which is irrelevant because the problem is sending to other Exchange Online tenants regardless of what the sending email platform is.

Is anyone else seeing this and have you been able to get to a solution with Microsoft?

Category: Email Security Appliances

Best Answer


  • Good to hear we are not the only one. Also looking at comments on downdetector it looks to be a wider issue and not just a few IPs so good to know it’s not our fault. Some reassurance and a place to point the finger when users ask about the delays and NDR’s.

    Microsoft support hasn’t once admitted that it could be a wider issue on their end and only try generic, tenant specific fixes.

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