Minor incident

KnowBe4: Defend - Inbound Mail Errors

Affected: Mail Flow

Current status of KnowBe4: Operational — live KnowBe4 status

Timeline

1. Oct 31, 2026, 11:22 UTC
   monitoring
   We have identified an issue that is causing some inbound emails for KnowBe4 Defend customers using Exchange Online connectors to be rejected. We have implemented a fix and are monitoring.
2. Nov 2, 2026, 12:56 UTC
   resolved
   This incident has been resolved.
3. Nov 5, 2026, 15:07 UTC
   postmortem
   From Thursday, October 29, 2026, at approximately 08:53 \(UTC\), to Saturday, October 31, 2026, at around 11:20 \(UTC\), some customers had inbound email rejected by Defend, and senders received a non-delivery report. This issue was caused by a software update to the Defend mail relay that applied stricter validation to long sender addresses. As a result, some valid inbound messages with long sender addresses, such as notifications and reports, were rejected. To resolve this issue, our team reverted the update. Defend mail flow returned to normal performance by about 11:20 \(UTC\) on October 31, 2026. We advise senders to resend any messages that were rejected during this period. On Monday, November 2, 2026, from approximately 13:40 to 13:45 \(UTC\), a follow-up update for the initial issue contained an error in its message processing, leading to temporary delays of up to 15 minutes for inbound email. Our team quickly detected the error and reverted the update. The sending mail servers retried affected messages automatically, so senders didn't need to resend them. To prevent this type of issue in the future, we're improving our monitoring and alerting for Defend mail relay errors so that we detect rejected email sooner. No data loss occurred as a result of these issues.

Updates as published by KnowBe4 (original).