26/11/2021 10:54:00 AM (GMT)
Issue Reported : 25/11/2021 12:33 PM (UTC)
Microsoft Reference : EX300267
Issue Status: RESTORED
Issue Description: Users are unable to send email messages due to multiple on-premises IP addresses being blocked.
More info: Users may receive a Non-Delivery Report (NDR) when trying to send email messages. The offending update was around 10% deployed before it was identified to be causing impact and halted. While many tenants were not affected by this event, a subset with on-premises IP addresses were significantly impacted and could not send email. While we worked to resolve the event, affected tenants could contact our support team and request that we manually unblocked affected IP addresses.
Scope of impact: Some users leveraging on-premises IP addresses to send email messages may have experienced impact.
Final Update – 02/12/2021 07:43:00 AM: Microsoft confirmed that a recently deployed rule change meant to improve anti-spam detection was mistakenly placing legitimate IPs on the block list, resulting in impact. They reverted this rule change and restored the block list to a previous and accurate state, remediating impact.