Routing guest VLAN requests to your internal servers the correct way

I have 2 vlans, one for staff and one for guests. I tell staff to connect their phones etc to the guest network. The DNS on staff is our internal domain controllers, everything works fine, the email server mail.consoto.com resolves to the internal ip and all is well. However if people connect to the other network, I have google dns setup for the guest network, mail.consoto.com will resolve to the external IP of the mail server, however it will not route traffic to that IP. I checked firewall logs and cant see anything, so this leads me to think I need a static route of some kind to tell the firewall to route requests from guest correctly? Everything else on the guest network works fine, its just used for internet access. any pointers would be great guys!