Zimbra 8.6 and Mac OS X 10.6

[mafiabusiness]

I have a client using Mac OS X 10.6 and when they attempt to add the CardDAV account into their Apple Addressbook, via CardDAV, they get a message saying

"cannot verify your account, please check details with sysadmin" (which is me).
I can log on to the webmail with same detais.
I look in the system.log of that Mac, and is proclaims that there is an SSL error when trying to handshake with the server.
I upgrade that very Mac to Mac OS X 10.7, and I can add the account without problems.

The same client use CalDAV (Calendar) and IMAP (email) over SSL without any problems.
It is just CardDAV that is troublesome.

Any ideas?

Miguel