Oh, different problem. If you want to be able to access OWA from the inside using the external DNS name, you're going to have to make an A record on your internal DNS server pointing to the internal IP address. (And if you're going to do that, making a zone for the public domain name on your internal DNS server, you'll have to replicate all the other public records or your users won't be able to get to, for example, 'www.companyname.com'.
Or just create an internal A record like 'mail' and get a multi-name cert that will work with both the internal and external domain names.