Public domain blocked by firewall

We have a fortigate 200b and can't figure out why we can't access our public domain dns addresses internally on the network, the active directory domain is similar the public domain. It resolves properly but it's like it gets blocked. For example, the active directory domain is company.domain1.com and the public domain is domain1.com. When we ping the dns record internally It resolves correctly but the access gets blocked. Accessing it externally it works fine. We have to set the dns on the firewall to point the public dns record to resolve to the internal server address for it to work internally. It's like this for any dns record we setup for our public domain. Any ideas why?