It looks like the object class for user accounts is 'zimbraAccount'. It has lots of attributes, but some of the usual suspects are there: cn, uid, givenName, sn, mail (thought there's also zimbraMailAddress, I don't know which one stores the actual email address), co (country), l (city), st, (state), company, postalCode, telephoneNumber, etc.
I hope this puts you in the right track and I don't need to setup a full Zimbra install
Saludos.
Iñaki.