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Today, some Microsoft 365/Exchange Online customers reported that they are not able to send emails to Microsoft 365/Exchange Online email IDs. Microsoft confirmed this issue and mentioned that users may encounter delays in receiving external email messages in Exchange Online, and they are working to resolve it.
The root cause of the problem is that specific IP addresses are being unexpectedly blocked by Microsoft’s own anti-spam rules, causing inbound external email delivery to become throttled and delayed. Microsoft is now analyzing whether any recent changes to its anti-spam rules are causing the problem. For now, Microsoft is manually adding reported affected IP addresses to an allowed list to provide immediate relief.
It’s an ironic situation where Microsoft’s measures to protect its users from spam end up blocking its own services.