Widespread Microsoft 365 IP Block Causing Delivery Failures for Multiple Users

Update timeline

1. monitoring Aug 15, 2026, 01:31 AM UTC

   Beginning Aug 14 2026, ~21:45 UTC, we have observed an increase of 10-15% in blocks from Microsoft 365 impacting all Twilio Sendgrid IPs. We have confirmed with other industry contacts that they are observing similar behavior. The blocks are being observed at many domains hosted by Microsoft 365 and are not confined to a specific subset of recipient domains. The blocks are variation of the following: 550 5.7.511 Access denied, banned sender[x.x.x.x]. To request removal from this list please forward this message to [email protected]. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653. AS(1410) Microsoft status page is reporting all systems are operational https://status.cloud.microsoft/m365/referrer=serviceStatusRedirect Impacted users can submit delisting requests per the instructions provided by Microsoft by sending an email with a copy of the error message, and the blocked IP address, to "[email protected]". We will be monitoring and providing updates as new information becomes available.

2. monitoring Aug 15, 2026, 01:37 AM UTC

   We are continuing to monitor for any further issues.

3. monitoring Aug 15, 2026, 01:53 AM UTC

   We are seeing reduction in block rates and some customers are indicating improvements. We will continue to monitor for any further updates.

4. resolved Aug 15, 2026, 02:05 AM UTC

   The incident is resolved and block rates are back to normal but customers will see change in block classifications from Microsoft 365. Users that continue to see elevated blocks can submit delisting requests to Microsoft by sending an email with a copy of the error message, and the blocked IP address, to "[email protected]". Delisting requests can also be made via the Office 365 Anti-Spam IP Delist Portal at https://sender.office.com/