If Mail on your Mac can’t decrypt emails
If you receive an email message that the Mail app on your Mac can’t decrypt, it’s possible your digital certificate or its private key is missing from a keychain. Try these suggestions.
If you have a backup of your keychain (created when you export your digital certificate and private key), import the Personal Information Exchange (.p12) file. See Import and export keychain items.
If your digital certificate and private key are stored on a Common Access Card (“smart card”), insert the card in your Mac, then try again.
If you no longer have your digital certificate or its private key, obtain a new certificate. See Use personal certificates.