macOS Apple Mail unable to decrypt s/mime encrypted messages from Windows Outlook

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Level 1

4 points

macOS Apple Mail unable to decrypt s/mime encrypted messages from Windows Outlook

I have a standard set of PIV certificates using RSA keys.

Messages composed and encrypted in Windows Desktop Outlook can be decrypted in Thunderbird, Kmail, and Outlook. Only Apple Mail fails to decrypt the message.
Messages composed and encrypted in Apple Mail can be decrypted in Apple Mail, Thunderbird, Kmail, and Outlook.
Message composed and encrypted in Apple Mail, Thunder, or Kmail can be decrypted in Apple Mail.
Messages signed in Windows Desktop Outlook are verified fine in all email clients, including Apple Mail.

This occurs on the macOS Monterey (Apple Mail 16.0) and macOS 15 Sequoia (Apple Mail 18.0).

On Outlook, I've tried different configuration including various encryption algorithms, including/non-including certificates, ... None made a difference. All email clients continued to work except for Apple Mail.

Anyone else experiencing this issue? Any recommendation on what I can look at?

MacBook Pro (M2, 2022)

Posted on Jun 27, 2025 01:38 PM

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Answer 1

jplejacq Author

Level 1

4 points

Aug 1, 2025 10:38 AM in response to jplejacq

I submitted a bug report using the Microsoft Feedback Hub app.

https://aka.ms/AAwy6u9

I also submitted a question in the Microsoft Community forum. I've gone back and forth with a Microsoft representative but nothing helpful as of this date.

https://answers.microsoft.com/en-us/outlook_com/forum/outlk_win-outtop_classic-outsub_ofb/smime-issue-unable-to-decrypt-message-in-apple/99e064b9-84e9-4840-8845-129bf450638f?messageId=37fbb37e-3ecf-449d-87a3-c89ae45dd471

Finally, I submitted a bug report to Apple using the Feedback Assistant app. Apple does not provide a link to a public website. They did assign the following ID to the bug report. I've received a request for more information, which I provided., but nothing helpful as of this date.

F**********8

[Edited by Moderator]

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Answer 2

jplejacq Author

Level 1

4 points

Aug 1, 2025 10:15 AM in response to jplejacq

This issue continues to exist for updated versions of macOS, Windows, Apple Mail, and Outlook.

Windows OS Name: Microsoft Windows 11 Pro

Windows OS Version: 24H2 (OS Build 26100.4770)

Outlook: Windows Desktop Outlook Classic

Outlook Version: 2507 (build 19029.20136 Click-to-Run)

Mac OS: Sequoia 15.6

Mac OS Version: 15.6 (24G84)

Apple Mail: macOS Apple Mail

Apple Mail Version: 16.0 (3826.700.81)

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