Mac Mail certificate suddenly not trusted

Hey there Michael Vallance,

We recommend using the guidance below to help with untrusted signatures, then follow the steps for trusting sender's certificates which is also linked below:

•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.

If Mail on your Mac can’t verify a signed email - Apple Support

Trust a certificate on Mac - Apple Support

Trust a certificate on Mac

If the Mail app  on your Mac can’t verify a server or a signature, you can review the certificate that it displays.

1. In the Verify Certificate dialog, click Show Certificate.

2. Review the certificate, then if you have confidence in the server or signature, select the checkbox to always trust the certificate or to acknowledge the signature is valid.

3. To read about current trust policies, click the arrow  next to Details. See Change Certificate trust policies.

Click Connect or OK, then enter your account password (if required).

4. If the certificate is a root certificate, enter the name and password of an administrator of the Mac you’re using.

Take care!