10-05-2021 09:22:12 AM (BST)
Issue Reported: 07/05/2021 05:42:00 PM
Microsoft Reference : EX254972
Issue Status: RESOLVED
Issue Description: Users get NDRs when sending messages from new domains recently delisted via the Office 365 Anti-Spam IP Delist Portal. A service update changed the method Exchange Online Protection serializes messages causing a contention with how they’re deserialized after being delisted, resulting in impact
Scope of impact: Any user sending messages from new domains that were recently delisted may be impacted.
Current Status: The deployment of the fix has completed and Microsoft have performed tests to confirm that it successfully resolved the issue.