Mail for Mac: If Mail can’t decrypt messages

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. For more information, 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. For more information, see Use personal certificates in Mail.