In the Root section of the S/MIME certificates panel you can view root certificates. Root certificates are the public certificates of certificate authorities that we trust. For instance, if an e-mail is received by the system with a certificate issued by trust authority A, DME scans the list of trusted authorities for A. If A is not found, the certificate is rejected, and the e-mail will not appear to be signed when opened by the recipient.
Just as user certificates, you can upload trusted root certificates in this section of the S/MIME certificates panel.