Circling back - we eventually managed to escalate to Exchange Online support. Since it wasn't our "tenant" that had the problem, it took a while.
Standard answers of "this is a rate change problem", asking if the IP was new. After showing them 30 days of traffic statistics, they reset the throttle but could not provide any reason why it got listed. Graeme From: Graeme Slogrove <[email protected]> Sent: Tuesday, 4 May 2021 4:09 PM To: [email protected] Subject: [mailop] Sudden deferred deliveries to Office365 Hi, At exactly midnight UTC, one of our servers started receiving the error 451 4.7.500 Server busy. Please try again later from [20.40.161.177]. (S77719) The error codes seem to indicate a problem with IP throttling due to changes in traffic patterns, but our logs show less than 2% variance in traffic to Office365 from that IP address Any ideas who to contact? Regards, Graeme Slogrove Product Engineering This transmission may contain information that is privileged, confidential, and/or exempt from disclosure under applicable law. If you are not the intended recipient, you are hereby notified that any disclosure, copying, distribution, or use of the information contained herein (including any reliance thereon) is STRICTLY PROHIBITED. If you received this transmission in error, please immediately contact the sender and destroy the material in its entirety, whether in electronic or hard copy format. This transmission may contain information that is privileged, confidential, and/or exempt from disclosure under applicable law. If you are not the intended recipient, you are hereby notified that any disclosure, copying, distribution, or use of the information contained herein (including any reliance thereon) is STRICTLY PROHIBITED. If you received this transmission in error, please immediately contact the sender and destroy the material in its entirety, whether in electronic or hard copy format.
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