Emails May be Delayed to Office365/Exchange Online Tenants

Microsoft has resolved this issue.

Further Update from Microsoft Support

Current status: Our review of simple messages has lead us to a potential root cause where a portion of SQL infrastructure may be receiving an unusually large number of requests that may be hindering Exchange Online's performance. We're working to confirm our findings to cultivate further troubleshooting steps. Scope of impact: Your organization is impacted by this event, as well as users who are attempting to receive external email messages within Exchange Online.

Customers may experience some email delivery delays from our network to Office365 based recipients.

Microsoft have reported the following issue on their network.

Title: Some users experience delays receiving external email messages within Exchange Online User impact: Users experience delays receiving external email messages within Exchange Online. More info: Affected users will see a "451.4.7.500 ServerBusy" error message. Current status: We’ve identified that specific IP addresses are being unexpectedly limited by our anti-spam procedures, causing inbound external email delivery to become throttled and delayed. We're reviewing if there have been any recent changes to our anti-spam rules to understand why the IP addresses are being limited. In the meantime, we're manually adding reported affected IP addresses to an allowed list to provide immediate relief. Scope of impact: Your organization is impacted by this event, as well as users who are attempting to receive external email messages within Exchange Online. Next update by: Wednesday, October 11, 2023 at 8:30 PM GMT+8