Cannot get to OWA from within the LAN

mail.company.com points to W.X.Y.Z according to a DNS zone we have access to at the ISP

W.X.Y.Z is the external IP of our Fortigate FW.

Outside of the LAN (ie at home), I can get to https://mail.company.com/owa

But within the LAN,  I cannot get to it using https://mail.company.com/owa.  I can however get to it using https://exch2010/owa.

Any ideas why?

I'd imagine if i create a company.com zone on our internal DNS servers on the LAN and point "mail" to the internal IP of the exchange server, it would work?  But this is not something I want.