I am planning a full install of Zimbra to get ready for some testing and then hopefully migrating and moving to production. I have a question around the need to provide a split horizon or split brain DNS.

In our situation we leverage a third party (our ISP) to handle our public DNS queries and have only an internal DNS for our local lan. This DNS also cache's outside queries. I am able to have the third party DNS handle the MX records.

If I continue to leverage the third party DNS then I don't believe I need a split horizon DNS on the inside. Am I correct or is there something I am missing?