When I open a mail that contains some graphic elements stored on a SSL protected URL, I get a warning window stating that :

"external_host" uses an invalid security certificate.
The certificate is not trusted because it is self-signed.
(Error code: sec_error_unstrusted issuer)

I guess that I should add the certificate of the external host to the keystore of Zimbra Desktop.
But how to do it?

I'm running Zimbra Desktop 7.2.2 under Mac OS X Snow Leopard (Java JRE 1.6)