Asa and Dns

We have recently switched to an asa 5500 series from another manufacturer's firewall. I previously had an A record in dns(microsoft AD dns)that mapped to our mail server(for owa access). After our firewall migration to the pix/asa i've noticed that this A record no longer worked and i had to access owa internally via ip address. To resolve this i'm having to create a local dns zone(with multiple records). I'm wondering why i need to create a local dns zone to get this resolution to work with the Asa. On my previous firewall i didn't need to do this. Thanks