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 […]