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.
In the Verify Certificate dialog, click Show Certificate.
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.
For information about current trust policies, click the Details disclosure triangle. To learn more about trust policies, see Certificate trust policies.
Click Connect or OK, then enter your account password (if required).
If the certificate is a root certificate, enter the name and password of an administrator of the Mac you’re using.
If you want to change a certificate’s trust policy later, use Keychain Access.