Intermittent DMARC failures for messages to Microsoft-hosted inboxes

Update timeline

1. monitoring May 01, 2026, 03:52 PM UTC

   In April, some customers may have experienced intermittent DMARC failures on messages to Microsoft-hosted inboxes. Impact was most visible for senders with a DMARC policy of p=reject or p=quarantine. The root cause was related to how certain email headers were being handled. We have implemented a fix and will continue to monitor closely. While there may still be some residual impact to messages as outstanding retries and queue delays clear over the next ~72 hours, we expect impact to remain limited.

2. monitoring May 01, 2026, 04:34 PM UTC

   In April, some customers may have experienced intermittent DMARC failures on messages to Microsoft-hosted inboxes. Impact was most visible for senders with a DMARC policy of p=reject or p=quarantine. The root cause was related to how certain email headers were being handled. We have implemented a fix and will continue to monitor closely. While there may still be some residual impact to messages with elongated headers and as outstanding retries and queue delays clear over the next ~72 hours, we expect impact to remain limited.

3. resolved May 02, 2026, 07:31 PM UTC

   HubSpot has continued to monitor the fix and believe we have addressed the underlying issue. Some messages deployed before May 1, 5 pm EDT may still experience delayed deliveries or failures over the next ~48 hours as retries and queues fully clear.