Customer.io incident
Reduced Deliverability to Microsoft-Hosted Email Addresses
Customer.io experienced a notice incident on September 4, 2026 affecting Message Sending, lasting 1h 19m. The incident has been resolved; the full update timeline is below.
Affected components
Update timeline
- investigating Sep 04, 2026, 12:16 PM UTC
We are currently investigating reports of reduced deliverability affecting messages sent to Microsoft-hosted email addresses. Our team is actively looking into this and we will provide an update as soon as we have more information. Thank you for your patience!
- monitoring Sep 04, 2026, 12:30 PM UTC
Starting around 08:00 UTC, Microsoft began experiencing an incident causing an increased volume of deferred/bounced emails. We are actively monitoring the situation. No other email providers are currently affected.
- resolved Sep 04, 2026, 01:35 PM UTC
Microsoft has resolved the underlying issue, we are seeing deliveries to their services resume as normal. Earlier bounces will be retried accordingly. We appreciate your understanding and please do reach out to support if you need any further assistance.