Download and install the Mac OS X v10.5.8 Update manually or via Software Update.
Mac OS X v10.5.7: Cannot verify identity of website using custom Certificate Authority in Safari or Mail
In Mac OS X v10.5.7, if you install a custom Certificate Authority in the System keychain and enable the option to "Always Trust", Safari may not trust websites using certificates signed by this Certificate Authority. When visiting such a website you may see the alert "Safari can't verify the identity of the website".
You may see a similar message in Mail when connecting to a mail server with SSL enabled, if that server uses a certificate signed by this Certificate Authority.