Microsoft 365 Alert – Service Degradation – Exchange Online – Some users may be unable to open inbound Secure/Multipurpose Internet Mail Extensions (S/MIME) encrypted email messages – ONGOING

08/12/2025 09:31:00 AM

NHSmail Reference: INC46699718

Microsoft Reference: EX1192586

Issue Status: ONGOING

Issue Description: Users may be unable to open some inbound S/MIME encrypted email messages in Exchange Online.

More info: This issue strictly impacts users with the Dynamic Delivery policy enabled.

Microsoft have disabled Dynamic Delivery “for S/MIME messages” while they work to resolve this issue.

Additionally, some users see a “Safe Attachments Scan in Progress” message upon opening an encrypted inbound email message.

Current Update: 08/12/2025 09:35:00 AM – Microsoft internal service telemetry has validated that their fix is addressing the problems, and that users have been able to open all new inbound S/MIME encrypted email messages since Friday, December 5, at approximately 9:40 PM UTC. Following their completed fix deployment, they are enabling the recovery of the previously undelivered messages, which is expected to be complete by their next scheduled communication update.

Scope of impact: Some users with Dynamic Delivery enabled attempting to open S/MIME encrypted email messages may be impacted.

Root cause: A recent service update that intended to introduce a new Microsoft Defender for Office 365 feature incorrectly marked S/MIME messages for delivery, resulting in impact.

Next update by: Monday, December 8, 2025, at 6:30 PM UTC