External users’ replies to encrypted email messages may not be delivered in Exchange Online

Timeline (UK time)

1. 30 Oct 2026, 22:21 GMT ✓ Operational

   Title: External users’ replies to encrypted email messages may not be delivered in Exchange Online User impact: External users’ replies to encrypted email messages may not have been delivered in Exchange Online. Final status: We've applied an exclusion to the affected domains and confirmed after monitoring service telemetry that errors associated with failed deliveries have ceased, confirming impact resolution. Scope of impact: This issue may have affected any users utilizing encrypted messages. Start time: Thursday, October 29, 2026, at 4:00 PM UTC End time: Friday, October 30, 2026, at 10:11 PM UTC Root cause: An anti-spam rule was being triggered on internal domains associated with encrypted message delivery, resulting in impact. Next steps: - We're reviewing our anti-spam rules to better understand why they were triggered on the affected domains and prevent similar future occurrences. This is the final update for the event.

2. 30 Oct 2026, 21:59 GMT ✓ Operational

   Title: External users’ replies to encrypted email messages may not be delivered in Exchange Online User impact: External users’ replies to encrypted email messages may not be delivered in Exchange Online. Current status: We’ve identified that an anti-spam rule is being triggered on internal domains associated with encrypted message delivery, resulting in impact. We’re applying an exclusion to these affected domains to exclude them from our outbound banned sender list to remediate impact. Scope of impact: This issue may affect any users utilizing encrypted messages. Start time: Thursday, October 29, 2026, at 4:00 PM UTC Root cause: An anti-spam rule is being triggered on internal domains associated with encrypted message delivery, resulting in impact. Next update by: Saturday, October 31, 2026, at 12:30 AM UTC

3. 30 Oct 2026, 21:47 GMT ✓ Operational

   Title: External users’ replies to encrypted email messages may not be delivered in Exchange Online User impact: External users’ replies to encrypted email messages may not be delivered in Exchange Online. Current status: We're investigating a potential issue with Exchange Online and checking for impact to your organization. We'll provide an update within 30 minutes.