Widespread Microsoft 365 IP Block Causing Delivery Failures for Multiple Users

What to do

• Check whether your product depends on Twilio. Check your stack

  Paste a package.json, lockfile, Terraform or a list of domains at down.guide/stack to see every provider you depend on.

• Confirm your retries, queues and scheduled jobs caught up after the 34m it lasted.

  Anything that failed between 2026-08-15 01:31 UTC and 2026-08-15 02:05 UTC may need a replay.

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Updates from Twilio 4

1. monitoringAug 15, 2026, 1:31 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 delist@microsoft.com. 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 "delist@microsoft.com". Delivery to Microsoft Outlook Consumer ("OLC"), for domains such as 'hotmail.com', 'outlook.com', etc., does not appear to be impacted. We will be monitoring and providing updates as new information becomes available.

   provider

2. monitoringAug 15, 2026, 1:37 AM UTC

   We are continuing to monitor for any further issues.

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3. monitoringAug 15, 2026, 1:53 AM UTC

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

   provider

4. resolvedAug 15, 2026, 2:05 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 "delist@microsoft.com". Delisting requests can also be made via the Office 365 Anti-Spam IP Delist Portal at https://sender.office.com/

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Corrections

None so far.

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