Apple Mail warning: Unable to Verify Message Signature

If Apple Mail displays the warning “Unable to verify message signature”, and when clicking details it says:

The digital signature is incorrect. The message may have been tampered with or corrupted since signed by (sender’s name).

the reason is due to a bug in Apple Mail (possibly introduced in a recent version of MacOSX).

See https://apple.stackexchange.com/questions/343251/apple-mail-warning-unable-to-verify-message-signature

“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.”