If Mail on your Mac can’t verify a signed email
If the Mail app on your Mac shows a banner stating that it can’t verify a signed message, it might be due to one of the following reasons:
The message may have been changed after it was sent. Ask the sender to resend it.
The sender’s certificate may have expired. Ask the sender to renew the certificate and resend the message.
You need to trust the sender’s certificate.
The email address on the S/MIME certificate doesn’t match the From address. The sender needs to change the From address so that it exactly matches what is used on the sender’s S/MIME certificate.
To view detailed information about why the message signature couldn’t be verified, click Show Details in the banner at the top of the message.