The version of Apple Mail distributed with macOS Sierra (released in September 2016) introduced a bug that prevents S/MIME-encrypted attachments from being opened or downloaded in Mail. The following steps describe a workaround to access attachments.
- Select the email whose attachment you wish to access.
- Select the email. Either in the left list of emails or – for threaded conversations in the left message area.
- Choose File > Save as to save the email as Raw Message Source
- Locate the saved email with Finder and double-click to open.
- You can now save your attachment.
- One of the first bug reports in Apple’s forum.