Symantec Messaging Gateway includes pre-installed CA certificates for many common certificate authority vendors. Add a CA certificate if a certificate authority issues you an SMTP/TLS or HTTPS certificate and that certificate authority's CA certificate is not already in the Control Center.
Another reason to add a CA certificate is if your certificate requires an intermediate CA certificate. When you add the CA certificate, you complete the certificate chain to permit authentication of a certificate authority-signed certificate.
Ensure that you have the CA certificate before you proceed. The CA certificate may have been included when you received the certificate from the certificate authority. Alternatively, you may be able to download the CA certificate from the certificate authority's website. The file that contains the CA certificate must be in PEM format.