Apple Mail warning: Unable to verify message signature
This seems to be due to a bug in Apple Mail, at least in macOS 10.14.1, but possibly introduced in macOS 10.13.5 as an attempt to mitigate the EFAIL security hole. 

When Mail is set to not automatically load remote content (Preferences > Viewing > Load remote content in messages is not checked), then the warning message Unable to verify message signature is displayed for every S/MIME signed message. 

You can get rid of the warning by either


Setting Mail to automatically load remote content (set Preferences > Viewing > Load remote content in messages to checked) not recommended
Clicking on the "Load Remote Content" button displayed on the right of the warning message


However, for security reasons, it is not recommended to load remote content unless it is from a trusted source.

I consider this a major bug for the following reasons:


It makes no sense that the verification of signatures is dependent on loading remote content, because the signature file is an inline attachment
There was no such dependency before
The warning is strongly misleading


I even consider this a severe security problem because:


Loading remote content is considered a security problem that can even lead to the leakage of encrypted content as described by EFAIL
The user won't be able to have the message signature verified without loading potentially dangerous remote content


The fix was originally intended to stop the Mail app from loading HTML content without the user's consent. However, the way it is implemented now, this leads to also not loading the signature attachment.