I installed Zimbra 5 and spent the entire afternoon in disbelief, reading more and more forum posts about zimbra not being able to authenticate microsoft AD users untill they are created in zimbra first. I'm not sure how it is done but we have successfully replaced outlook web access with squirrelmail, which has the option to authenticates against LDAP without the need to first import existing users let alone the need to sync both databases from then on... In squirrelmail you log on with your AD account and if it doesn't exist in the squirrelmail db it is created after successful authentication. Please provide some feedback as to when this feature might be fully implemented if at all any time soon so we may reconsider using zimbra to replace microsoft exchange. Thank you for an amazing open source product so far!!!!