Apple Mail cannot verify the digital signature, why??

Apple Mail cannot verify the digital signature, why??
10 Mar ‘23

On April 15th 2021, KeyTalk reported a problem to Apple, whereby under specific circumstances Apple Mail will not trust a digitally signed and/or encrypted email, when using the preview pane in Apple Mail.

This happens only when: The Apple Mail is used, mail is viewed in the preview pane, the mail arrives on an Exchange server, and when the mail is clear text digitally signed.

When the sender uses opaque S/MIME signing, the problem does not occur.

Pending a fix form Apple, the only way to ensure the digital signature can be verified by MacMail, is to open the full email (ie open it separately in MacMail, or have the sender send the email using opaque digital S/MIME signing.