We've got roughly 40k users on an outsourced Zimbra install. Most users are fine with the webmail versions, but some want to integrate other desktop MUAs. Their specific complaint is using the GAL to lookup emails from names and such. For random reasons, we don't have a good whitepages LDAP, and the Zimbra LDAP is not accessible. This could be changed, but I suspect it will take a good deal of convincing and coordination. Possibly lawyers.
In the meantime, is it possible to access the Zimbra GAL via a cardDAV interface? I've got Evolution working with my personal CardDAV contacts list, but that's a far smaller set than the entire 40k users.